Seattle Subaru Timing Belt Done Right.
So your Subaru is due for its timing belt. Your owner’s manual states replace the timing belt at 105k for most models produced after 1996. Most will call around looking for the best price and not having any idea it’s really not the same as shopping for a TV or a mattress. One shops timing belt replacement may differ greatly from the next and there is a right way and a cheap way. Ask questions beyond just the price, what does the service include, what kind of parts are being used? There is a difference and in the case of auto repair the difference usually shows up after the warranty on the repair is over. Understanding the repair should last fairly close to the amount of time the original part lasted is just as important as understanding the lower priced service may represent a lower value obtained.
The timing belt has a big job to do it must keep the camshafts spinning in time with the crank shaft. This is so the valves open up at the proper time in the combustion cycle in correlation with air and fuel coming in, compression and spark exploding the mixture and the exhaust valves opening up to release the leftovers. At the same time the timing belt needs to turn the water pump and be able to self adjust in current models. Over time the rubber stretches out above the tensioner’s ability to keep it tight under all load conditions and just like your tires the rubber belt will age, wear and crack and ultimately could break.
A worn or stretched timing belt can cause performance issues as well as increased fuel consumption, not in miles per gallon but in miles per tank.
Every vehicle manufacturer provides a warranty so long as the maintenance is done, every car company also has a breakdown of vehicle maintenance such as 30/60/90k service with a clear break down of what should be inspected and replaced at particular service intervals. But what is not understood is it does not look good to list a lot of items to be replaced. Meaning if you are Subaru or any other car company it doesn’t pay to be truly honest of all of the potential service needs because your car will be perceived to cost more to own than the next and a lot of people make buying decisions based on cost of ownership studies provided by JD Powers, and consumer reports to name a couple. Subaru and every other car maker leaves it up to whoever is servicing the car to inform or “sell the customer” on items not listed in the maintenance booklet. This is nothing new to the industry but is one aspect of maintenance if not done correctly can have some serious ramifications.
There are many different levels of parts quality, some shops will gravitate to the lowest cost part and as long as its new and lasts the standard warranty period of one year that’s all that matters to them. Next the skill level of Techs can vary greatly as well. If you are calling around for the best price on a service you really need to understand there may be a difference in the quality of the service your Subaru will receive having the timing belt done isn’t nearly as good as having it done completely with quality components by Technicians that really know what to look out for on your Subaru.
We approach this the right way, service your Subaru so there are no problems later by doing the job complete so there is no reason to go back in one or two years later. We offer a lower price than the Dealer service department but in some cases will be priced higher than a tire store offering a timing belt replacement. This is going to be a typical situation across the country, independent Subaru repair shops will typically be lower priced than a dealer service department but more expensive than “John Doe’s auto” and that is typically because we know the right parts to use and take pride in offering the best service possible. John Doe may not realize the green idler pulley is going to fail, or that there is a service bulletin pertaining to the timing belt tensioner and bracket.
The timing belt should be replaced at the 105k interval but the procedure to replace the belt is more complex than just replacing the belt. Part of the timing belt replacement procedure is to remove the timing belt and inspect the timing components. If after inspection any or all of the timing components are found to be questionable they should be done. This includes the timing belt tensioner, timing belt idlers, water pump, and cam and crank seals. The large expense of replacing the timing belt is the labor but once the Technician is behind the timing covers there are a lot of things that can be done to limit forward going ownership costs. We hear all the time about the dealer or another shop replacing the timing belt and leaving a tensioner to fail at a later date. This first picture shows what can happen when a timing belt tensioner is left to chance. This tensioner was ignored by a Dealer service department in another state only to fail 6 months later and the cost was significant as the timing belt came off and valve train damage was done. What could have been done for $152.00 or so (depending on the dealers mark up above list price) Turned into a $2000.00 valve job plus the cost of the tensioner, damaged timing belt and the time without a functioning Subaru.
The idea of replacing the timing belt only and thinking the tensioner or idlers are going to last another 105k without any issue is a gamble. I am not suggesting that every part needs to be done every time but in the case of the timing belt tensioner it really is a must replace in my opinion or you are just inviting a problem.
This is a picture of a first generation 2.5l that need to be inspected as part of a quality timing belt replacement procedure.
You can see that there is a lot more to a timing belt than just the belt, all of the components that are located behind the timing covers and that can affect the life of the timing belt all need to be inspected. Notice the way the timing belt is putting pressure on the water pump (the part with the hose attached on the right)
These are the components of a second Generation 2.5l The idler on the right has been superseded to the one in the middle but can still be used.
Next is a picture showing the difference between the O.E. Timing belt idler and one made by an aftermarket parts company. The original part used and the one superseded by Subaru call for a double bearing type idler and this one shown in the picture is a far cry from a part that “meets or exceeds” original parts manufacture.
No matter how you look at it, the parts in the two pictures above are not of the same quality and if you aren’t careful you could end up with the green one and worse for the same price as the one next to it. If it lasts the warranty period but not much longer it will cost more to correct it a second time.
There is no need to sacrifice quality when saving money. A good independent Subaru repair shop can save you money over a higher priced dealer service department and do much more for less. That is what we offer at All Wheel Drive Auto, complete repairs you can rely on.
Here is a Video of what we typically encounter when we remove the timing belt and inspect the timing belt components.
[...] Seattle Subaru Timing Belt Done Right. – Seattle Subaru Repair [...]
If the Subaru tensioner is not properly adjusted when belt is replaced can this result in damage to main bearings?
Timing belt and tensioners, head bolts,and your good gaskets may cost ? I guess I need the best products.
Rebuilt heads, and what will I need for the cams ?
I just purchased a 2001 Subaru Outback with 175000 on it. I’ve been thinking about replaceing the timing belt, but wasn’t sure of the importance after reading your posting I will be doing it in the real near future, I only wish you were in my area. With the info you provided I will know the questions to ask.
When these guys speak you should listen. They are the “Go To Guys” when it comes to Subie’s!
This article is 100% True. Many people wonder why the Subaru Dealer charges so much for a timing belt replacement but it’s because they are changing ALL the necessary parts, not just the belt.
The timing belt done at the dealer will cost you around $500-700 but at least it will be done right and your Subaru will run for another 100K with no problems. Small price to pay for another 100K in your Subaru.
I wish. My local dealer said $1750 for everything that goes with the timing belt replacement. The kit alone was 600. Wow.
Thanks for all this great info Jim! Just received a quote for a new timing belt (they said was needed on my 2004 Subaru Forester @ 134,000 miles)…no problem, vehicles need attention. My question to you is, the quote was $1,200…quite a bit more than your quote
What is your recommendation with this quote. AND what is a “Knock Sensor” also “recommended” by this shop.
I am having my timing belt replaced ( today 9/1/2010 , on my 2003 Subaru WRX w/2.0 turbo)
and the water pump ( the dealer recommended I do both at the same time) There was a problem getting the belt off because the bolt that holds it on to the cam shaft is put on with “lock tight” they had to break it off or it broke off while trying to get it off. So they said this happens 50% of the time and it will cost me another $89.00. Does this really happen 50% of the time? Should I have to pay for this? The bill is already $1100.00. It just seems odd to me that I should have to pay extra for something they broke.
Thanks,,
Ruth
Hi Ruth,
There is no bolt that hold the timing belt on.
There are 4 cam shaft sprocket bolts again none of which need to come off until the sprocket is removed to replace the cam seal or something along those lines.
So I am not quite sure what is broken or why.
But I will say that things can and do come up during repairs that there is just no way to know about ahead of time and as frustrating as it can be its kind of the way it is, I would ask to see the parts and get a clear explanation you can understand while viewing the broken component.
Justin
I had my timing belt replaced three weeks ago from the day I started the car to hear a horrific noise. Had it towed back to the shop that replaced the timing belt. Now I have two bent valves. Could these be related? I have maintained my 2002 Subaru Forester rather well so it doesn’t make sense to me. Thank you!
Hi Penny,
You just dont have enough information there to know what happened such as why does it have 2 bent valves?
Did the timing belt jump time? If so why?, At the time the timing belt was replaced were you given the option to replace the timing belt tensioner, idlers and water pump , if so did you decline these repairs.
If not, why were they not offered to you and did any of these items fail resulting in the issues you now have?
Most likely the timing belt jumped, the valves are now bent as a result, We just don’t know the why at this point.
Justin
I have a shop that will replace the timing belt and tensioner for 500 bucks. They say it includes everything that needs to be taken care of. What else should I ask? It seems like a good deal but, is it?
Hi.
I have a 2005 Subaru Impreza WRX wagon — it’s my baby, I love it. I took it to the dealer at 89,000 to have the 90k done and the timing belt replaced.
I took the car back after they finished because it was making an odd noise. They said that nothing was wrong, but they couldn’t explain the noise and didn’t recommend looking into it further. 10,000 miles later, my upper idler and my tensioner break, essentially busting the engine.
Was it at all possible that this could have been prevented? I take good care of my car, but now I’m looking at a new engine. I love my car, but I don’t see how 2 of the parts broke at the same time in the engine. Any help would be appreciated or any perspective on how or why this happened.
Thanks.
Cavan
Hi Don,
Not knowing the Model or year makes it tough to comment on. IF its an old legacy I would say sounds about right. Anything else is a red flag.
My guess is that it is all aftermarket parts.
Justin
Sorry…it’s a 99 Outback.
Great article. I wish you guys were in Central Jersey. The nearest Subaru dealer service quoted me $975.00 for all timing belt related job. I believe it’s still a fair deal but, I think you guys will be less expensive. I have a 2005 impreza rs (manual).
I am interested in purchasing a 2001 Legacy AWD 2.5L H4 MPI that is 108,000 miles old. I understand that timing belts are to be replaced at 105,000 miles. Would you provide a ballpark cost for timing belt, water pump, tensioner and idler replacement at All Wheel Drive for this vehicle?
Thank you for the information provided on your website.
THe Carfax shows a thorough service record, but the 105K service has not been addressed.
Looking forward to your reply.
Kelly
Hello Kelly,
Yes the timing belt is do at 105,000 As far as costs they vary greatly with exact needs.
I can send you an estimate with some breakdowns in costs for the basic timing belt replacement, plus revisions for idlers, tensioner, accessory drive belts and water pump.
Justin
Great information! I have a 2005 Baja Turbo with 42K miles. During a recent oil change at the Subaru dealership, I was told there was a loud noise coming from engine and the rep thought it was the tensioner as the noise was from the drivers side of the engine. (the technician didn’t have time to look at the engine at the time so I need to take it back). How can I tell if I am being mislead and what would cause problems so soon with the tensioner? The 30K maintenance was done less than a year ago with no problems.
Thank you.
Hi Jessica,
With out removing the timing covers, then the belt and inspecting the components there is really no way for sure to know what is wrong. I can offer that it is not uncommon for a tech to hear a noise you are unaware of though. The 30k would never address any timing component noise, or even for that matter an inspection of any of the timing components.
If it has been brought to you attention that you have a timing component type noise it could be a Gamble on your part to not Address it. You can pop the hood then start the vehicle cold, run back out and see if you your self can hear a noise that doesn’t sound quite right. Thats really all I can offer other than you making an appointment for a second opinion if you dont yet have a established relation ship with a service provider.
Hope that helps
Justin
Excellent advice… I can only add this: I had a lazy mechanic, who only replaced the timing belt… Guess what happened? A few years later, the tensioner pulley went out. As one of my buddies told me later; “You can’t always tell by looking if the tensioner is shot”. He always replaces the tensioner, all the pulleys, water pump and all the gaskets (parts which come in the complete timing belt kit) to avoid problems later… if only I had known!
Thanks for your comments.
Its great when someone takes the time to relay their experience to help affirm what I am always trying to say about the completeness of timing belt repairs.
Justin
Man what a great read. I am in Maryland. I think I am going to take a road trip to your shop and let YOU do my timing belt service. I used to live in Portland. I have been to Seattle many times. I can come there and hang out a couple of days while you work on my car!! What do you say?!?
>8^)
That Sounds We would be happy To service Your Subaru and provide you with a loaner car as well.
Let us know when you are ready
Justin
my 96 outback wagon with a 2.5 died and will turn over but not start…no engine codes…makes a buzzing sound with key in on position seems to be coming from my transmission…i am stumped…any help is appreciated
Jeff,
The Solenoids in the Auto Trans will buzz with the Key on and engine off, that is normal. It has to go through testing to determine what is missing, spark, fuel compression the ability to breathe or any of the above items at the wrong time in order to figure out why it wont start.
Justin
Justin, do you sell kits that have the better parts? If so, how much shipped to Harrison, Arkansas.
Hi Mark,
Yes we do under the Parts tab on our website?
Justin
I have a 98 legacy gt 2.5. I have some oil that is leaking from behind/around the timing belt cover. What could that be? I have 110k miles so it’s time for a new timing belt. Trying to understand the oil leak. Also just replace the PS pump as that was leaking some fluid on top of engine.
Hi Dave,
It could be one of the Cam seals, or the front crank seal? Its hard to say from here with out viewing it.
Justin
Justin,
How much should a timing belt change cost along with other parts for 2005 WRX Subaru Imp Wagon?
Thanks in advance.
Raymond
Hi Raymond,
If you are local to us I would steer you towards the request a quote page or call one of the service advisors.
If you are not it would be better to inquire locally as our price may not reflect what you will pay.
Justin
your info helped me out SOOOO much. the auto shop i just took my 97 outback to originally quoted me 600 for just the water pump and timing belt (which seemed reasonable) and then called back today and said it would be another 200 for some pulley’s as well. it doesnt sound as though im being ripped off (especially with my $100 off coupon
)but i sure hope these guys know what their doing. WHY OH WHY cant you guys be in SLC, UT!!!!!!!!
Glad our Video helped!!
Look for more as we have time.
Justin
Thanks for great reading, most interesting and superb explanation of the importance of replacing timing belts and the components around it!
Sorry that I am on the other (wrong) side of the Atlantic ocean!
(Sweden)
I own a 2005 Subaru Forester 2.5X, and it has 48,000 miles on it. Yesterday, I went in for an oil change at the dealership, and I was told that I needed to replace my alternator belt and a/c belt because they had developed cracks. And, I was encouraged to replace my timing belt as well because they said they use the alternator and a/c belts to gauge the wear-and-tear on the timing belt. I told them the manual recommended 105 months or miles, and they said it’s a different schedule for Houston because of the hot, humid weather. The cost of replacing all three belts would be about $600. I have no idea what to do, so any advice would be appreciated. Any chance y’all will be coming to Houston? We could really use a good Subaru shop down here!
Run
The timing belt does not need to be done yet and at $600.00 they are not doing the tensioner and or idlers which s a mistake.
Justin
Thanks for sharing your expertise! And, if you happen to know of any Subaru experts in the Houston area, I could use a good recommendation.
So I have a 2005 WRX and it’s at 84,000 miles. I’m about to drive from Fairbanks, AK to San Antonio, TX. The dealership suggested I change the timing belt before I drive out. However, now I’m scared after reading some of these stories. Should I just wait until I get to TX or trust the dealer to change it right?
Hello Dale,
At 84k the belt isn’t really due for another 20k, it is however tight on time but still a year off before I would really be concerned.
I will say however that I am not sure about the shops in Texas and Subaru, I dont think its a popular car there? That would be my only argument for replacing the belt in Alaska before the trip.
Justin
I have a 2005 Outback 2.5i, with 81,000miles. I was just told that I need to replace my head gasket by the Subaru service shop. They also suggested that since they would have the engine out, that I should have the timing belt replaced, in order to save on labor. W/o labor, they are quoting me $190 for the parts; which would otherwise be $1000. Do this sound right, and what do you suggest?
Hi Janelle,
Yes you should replace the timing components now with the head gasket repair, it will keep forward going ownership costs down and help prevent the feeling that your always putting big money into your car.
I will add that for $190.00 they are only replacing the belt and tensioner and I would strongly suggest the idlers as well, as well as the water pump. We have a bout one car a month towed in from the freeway that has had a timing belt idler fail 25,000 to 50,000 miles after a timing belt was done and caused some very serious damage. Use the search feature of our website and you will be bound to find some good strong information as to why you want to make the most complete reapir you can.
Justin
HI justin, your information is very helpful. i have a question. i have a 99 rs impreza manual with 80k, i recently got a quote for $399 from the subie dealer to get a timing belt change. they said that they will change my belt and inspect all my timing components. i am afraid that they will recommend unnecessary parts. what do you recommend what i should ask or do if they ask to replace this and that. any advice is deeply appreciated. thank you
Hi Ben,
$399.00 is the piece of Rubber and Labor. If any of the timing components fail later you will have defeated the reason for replacing the belt in the first place. At our shop we try to be just the opposite of the Dealer and thats HONEST. You should really approach this service as completely as you can, I know it costs more money that way and every one is on a budget but I would implore you to make a good complete service and clip coupons or the like for other items that are of less significance to make up for the increased expense.
We see at least once a month complete finical devastation brought on by an incomplete Timing belt service only to have a tensioner or idler fail later. If they aren’t done now than when? Search for idler on our website and you will see a few posts.
The problem with waiting to replace those other times is when traveling at 70 mph on the freeway when the idler seizes doesn’t give for any opportunity to get it done before something serious happens as its already happened. From the drivers seat its do difficult to audibly pick up an idler that is making some noise.
We are talking about bolt on parts that can be done for parts only prices while replacing the timing belt. I would insist that they be inspected and done as needed going into the timing belt job, and if the Dealer states they are ok and chooses to omit them, ask them for a life time warranty. The truth is that the Tech at the Dealer doesn’t want to bother waiting for you to agree to replace more parts that the tech wont get paid any extra to install, no one will help you if 2 years later one of the non replaced timing components fails and damages the engine. It happens, we see it and we dont like it!
I hope this helps
Justin
Thanks for publishing such an abundantly informative article!
If you don’t mind answering, I have a few concerns related to my situation…
My ’03 WRX is approaching 69k miles and is now 103 months old, with only exhaust and intake mods. (no engine or ecu tweaking) I’m now starting to plan for a timing component service/replacement. I believe the recommended interval is 105 months/105k miles. Which is more critical? Although my WRX is at the right age, I’ve still got 36k miles before the recommended “milage” service interval. I average 670 miles per month/8k miles per year. Would another year’s worth of driving be a reasonable expectation concerning the life of the timing belt?
A factor to note: I live in south Florida. Along with the high humidity, the air temps average in the high 80s most of the year, with mid 70s & 60s during the winter months. Does this type of climate affect the lifespan of belts? My power steering/alternator belt shredded about 3 months ago, with about 67k miles on it. Is that a warning sign that the timing belt may have a shorter lifespan down here in this hot, humid climate?
Thanks for your time! –>Buck
Hey Uncle Buck,
The original acc belts from the factory are actually going to be on there way out at 60k regardless of where you live.
You are correct in thinking about things in terms of age, we did recently have a 2002 Outback in with 59k but a failed timing belt. I also want to point out that one of the reasons or thoughts behind replacing the belt is that it does stretch and this will affect power and economy as well as tail pipe emissions. You could remove one of the outer timing covers and inspect the belt, but I do worry about the timing belt tensioner failing due to age just as much as the belt failing.
Climate does in fact affect the life of Rubber but in Florida( where I vacation yearly) it is more of a benefit VS say Alaska where its very cold and the rubber contracts greatly and then heats up due to friction.
I hope that helps and I wish I was in South Florida right now, its cold and rainy here today.
Justin
hello, so my idler went out today, my 99 impreza started making a grinding noise out of nowhere, pulled over and i couldnt figure it out. Had to the car towed home and pulled off the pulley cover to find the pulley bearing had shredded. Small balls and what i believe to be the inner or outer race in pieces. Will my valves be toast? Am I looking at a $2000 valve job? God, I hope not.
Hi Jimmy,
If the timing belt jumped or came off as a result of the failed idler then there is a high probability that the valves have become bent.
The only way to know is to repair the timing components and see how it runs, or rotate the camshafts until each cylinder( test one at a time) is on overlap, then do a leak down test.
Justin
Justin,
Great info here! I took my 2006 Outback wagon with the 2.5L (not turbo) to a local independent shop for the timing belt replacement. I didn’t know about replacing all the idler’s. He did replace one of them though. A couple of weeks later the bolt on that holds the tensioner in place broke on the interstate hwy. Still waiting to hear if I have valve damage or not. Have you ever heard of this bolt breaking? Is this a coincidence or is he not telling me something? Do you think I have bent valves?
Wishing you were in SW Michigan!
Dan
Hello Dan,
The bolt will typically break if its cross threaded in as it is weakened. I don’t know if that’s the case without looking at it however.
On a 2006 it most likely bent the valves I am afraid.
Sorry I cant offer more but I do hope that helps
Justin
I have a 2000 outback with 178,000. I’m sure that the timing belt was changed once before I purchased the vehicle as it had 145,000 milles at the time. I am about to take a trip from WV to Houston, TX. Would you recommend having the timing belt and all other associated parts changed before the trip? I have taken great care of the vehicle since I bought it and it had great service records prior to my purchase. Just wondering if you think it would hold up for the trip.
Thank You
Adam,
Unless you have a service record indicating the Timing belt has been replaced then its a gamble you are taking in assuming it has been done.
I would advocate to be proactive and have the belt and timing components done as the alternative is very expensive. You may be able to find someone that can have a look at the belt for a small fee, but there is no date stamp on the belt so it would still be a guess, I would also like to point out that the belt stretches and can cause performance and fuel economy issues and that’s another good reason to replace the belt, its not just for fear of breaking.
Hope that helps
Justin
Thank you for the speedy response. I will have it inspected ASAP at my local subaru dealership.
im doing this job now, allways replace all compnets,that the timing belt runs on. this is a 100000 mile repair, replace hoeses, drive belts, if cust. will let you put low friction engine oil in engine. HIGH QUALITY PARTS
another point rubber products have about 5 year service life, there are a few exceptions, heat , sun light, oils and differant fliuds
I have my 2002 subaru forester in the shop right now and they are quoting me $730 for timing belt, pulley, and tensioner replacement. is this reasonable?
Hello Nancy,
Sounds about right, but not seeing the estimate I dont know if its complete.
Justin
Wow. Good post. I own an Outback and the very problem you described in your blog/video just happened. At about 200K the belt jumped 3-4 teeth because an idler seized up. Luckily, no valves were bent but it still cost me almost $850. Thanks again for your post and I feel for those who have no idea whats happening under their hood. It’s a costly lesson sometimes!
When replacing the timing belt, a lot of other things should be replaced as well!
Justin,
You were correct, there is valve damage. Serious expense because of a broken bolt. I have had three different mechanics tell me the bolt broke because it was over torqued. I looked at the extracted part of the bolt and I saw no evidence of cross threading. I have moved the car to another mechanic. One who has his torque wrench calibrated every 8 weeks. We are replacing all idler pullys, the tensioner, the belt and the bolts. Everyone who reads this, please listen to Justin, replacing the heads is about $1000 (if you can find them) and the labor is another $1100. It is always best to do this job right the first time. I have been without a car for over 3 weeks now and I drive 40 minutes to work one way, what a pain.
Dan
I live in Southeast Alaska and have a 99 Forester with 105,000 miles. The car is as noisy as a diesel when cold but quiets to almost normal when warmed up. I assume it has had piston slap for at least 20,000 miles. I figure I could wait for something to fail before I take any action. Does this make sense? What would be your recommended repair? Short block?
Thanks for a great site.
Paul
I don’t generally suggest correcting piston slap unless its there after the car warms up and never goes away. We install Reman Subaru and New Subaru Short blocks that have piston slap as well, its hard to know how severe yours is to speak as to how to advise you going forward. . Generally speaking piston slap poses no detriment to the engines longevity. We rebuild our selves in house as well and have good luck with Top line pistons being quiet.
Justin
Justin,
Thank you for your considered and helpful reply. It looks like I should be planning on the timing belt replacement then if the piston slap isn’t more likely to damage the engine first. Perhaps a trip on the ferry to Seattle is in order!
Paul
Justin,
I’m replacing head gaskets on my forester. I pulled the TB tensioner off to compress the piston and noticed the threads came out with it. Do you know what size heli-coil kit I need to repair it?
Justin,
I’m the guy with the broken TB Tensioner bolt. The second mechanic pulled the heads without removing the motor. They rebuilt the heads and reinstalled them. Now I’m getting a Check Engine light with a P0420. They replaced the mass air flow sensor and both O2 senors and still have the P0420 code. To make matters worse, I’m driving it the other day and “cap” that fits over the coolant supply to the turbo ruptured (my engine is not turbo). I sure hope I got stopped in time to prevent serious engine damage. The engine is an EJ25 in a 2006 Outback.
Question 1: Have you ever seen this rubber cap rupture?
Question 2: Is it possible that they damaged this rubber cap while Removing and replacing the heads?
Question 3: Any thoughts on this work and how it might have contributed to the P0420 code?
Thanks
Dan
Dan,
I don’t remember the broken timing tensioner bolt post I am afraid. P0420 is common after HG repairs and its really just pointing to a component that was marginal prior. If a drive Cycle wont correct the issue, if the catalyst fails a catalyst performance test It really needs a catalytic converter, and while I understand that maybe the light wasn’t on before, and the code wasn’t set, the battery was disconnected and learned data lost and that’s not really anyone fault or responsibility. I will point out that a Catalyst Efficiency code is Conditional and not a component code meaning that a condition is not present or is present the ECM doesnt like, its possible that it may get better with time as the most common cause is lack of Catalytic convertor heat. Read more here about p0420
But since you asked, if the “mechanic” didn’t remove the exhaust merely dropped it down, some of the coolant form the head coming off could have made its way into the exhaust, and Catalyst this would not have helped a marginal component. Now I don’t know if that’s what happened, as it doesn’t have to happen to have a p0420 post head gasket failure and subsequent repair.
I am not sure which rubber plug you are referring to.
Justin