Your Subaru Check engine light explained part II.

If you haven’t already read the post your Subaru check engine light explained I would encourage you to, as I am only going to try and cover things not already talked about or will be expanding on the previous article.

I have received a lot of comments about the previous post and as I answer Question under Subaru Repair on all experts everyday as well as talking to our local customers.  I have a good feel for what is going through the minds of many Subaru owners wherever they may live. 

The first thing I really want to stress here is that a good thorough analysis of a codes set in regards to a check engine light is always better than plugging in a code reader at a parts house. You can go to the doctor and have your blood pressure checked and they can inform you it’s high, here are some pills or you can have further tests done to determine the cause of the condition and any symptoms.   Knowing the code set in the ECM’s (Engine Control Module) memory is like knowing you have a fever, when why is the answer that is needed.  The choice is really yours to make and a cheap or free check engine light scan does not necessarily equal value.  There is the old saying you get what you pay for.  

Next there are hundreds of possible codes, thousands of possibilities and only one light, can you imagine an airplane having one light for all of its many engine management systems?  There should be a check engine light pertaining to the engine, its sensors and control devices necessary for proper engine function and another light for Emissions control devices and the systems that monitor those devices.  But that is not the system we have nor does it appear that this is going to change anytime soon.

It is far more complicated than most understand, any many cars go without ever being properly diagnosed and repaired.  The biggest area of confusion is that some believe the light comes on at a preset interval, but the reality is there is no check engine light on your 1996 and newer Subaru that comes on at a predetermined mileage or time frame. The second is that some times the light will go on and off for months as a result of programming, monitors, drive cycle events, temperature and the nature of electricity.  Most of the time the car will seem to run ok and that is a result of what is called adaptive strategy, this is the ability of the ECM to overcome faulty sensors, intake leaks and even some misfires.   

I would say that less than 35% of the industry actually understands how an OBDII system actually works and I guarantee you that not one person behind a parts counter with a code reader understands what a catalyst monitor is and what are the parameters of the monitoring system.  So why should they give any advice at all in regards to a check engine light, is there any value in there opinion?   In the portion of states that have a vehicle emissions programs the Technicians that are licensed by the state to diagnose and repair emission related problems are a leg up on Technicians that are not required to maintain there license and thus their education.

The Check Engine light is more about the Emissions systems on the car then it is the actual engine itself.  If the Technician working on the car is not an Authorized Emissions Specialist he or she may not have received the proper training or is defiantly not going to be as up to date as a Technician that is staying current.  I often find that when I talk to my customers about what is truly involved in the diagnoses and drive cycle after repair, most had no idea just how complicated it can be to truly diagnose, repair and than verify that repair.  

The check engine light comes on and a code is stored in the computers memory, the engine may or may not feel any different.  this is really where the confusion starts.  A loose gas cap can cause a “check engine light” but what on earth does the gas cap have to do with the engine?   I wish there were two lights and two legs to the the system one being a check engine light and the other being the check emissions light.  But the federal government was afraid of creating confusion with multiple systems, yes in 1996 The government decided we needed a standardized system. I agree that we needed some standardization , but the intent was to clean up the air and this attempt has largely failed.  The single biggest problem is lack of enforcement.  I hear so often that a Dealership told a driver the light was no big deal when the real secret is there are a lot of items on a newer car that are under warranty for a much longer period of time than most drivers are aware of.  Or I hear about a check engine light being on for three years with only maybe some confused partial attempt at repairs usually done by scanning it at the local parts store and installing one or two suspected “bad ” parts.      But there is only one light and there is no way to know if the check engine  light is on because of a loose gas cap or a failed coolant temperature sensor without a proper diagnoses  or at a minimum a code scan, one will let excessive hydrocarbons into the atmosphere in the way of raw fuel vapor and the other may leave you stranded.  

Here are some tips for 1996 and newer Subaru vehicles that will hopefully help.

On the dashboard of your car there are many different colors of light.  Lets break down the different colors and what they each mean. The turn signals are green and just like a traffic signal green means go.  The check engine light, ATF temperature light are yellow and yellow means proceed with caution.  The oil light & battery light are red and mean stop driving the car.  The additional aspect of the check engine and transmission temperature lights is the ability to flash which is basically an indication to stop driving the car.   If your check engine light starts flashing you need to have the car looked at as soon as possible.  

Consider purchasing a basic code scanner yourself if you have a diagnosed emissions related problem that you cannot afford to repair.  For example a Catalyst system may cost upwards of $1200.00 to repair and that could be a hard pill to swallow.  But since there is only one light how will you know if anything comes up? This is when I suggest a basic code scanner, if something new comes up on a weekly or biweekly scan you will know to have it diagnosed.   Keep in mind a code is only the starting point.

There are two different types of codes component and conditional codes, here are some of the most common codes we deal with and the process that needs to be done to truly get to the bottom of the code.

P0440 Evaporative emissions leak.  The ECM monitors the Evaporative system pressure and through the monitoring system believes there to be low or no pressure, the most common cause is a loose gas cap but a leak any where else in the system can cause this as well, typically a smoke generation machine is used to look for leaks in the Evaporative Emissions system A smoke machine can cost over $1000.00 and has on going cost in the way of the what ever is used to create the smoke such as mineral oil.  So why on one hand a loose gas cap is pretty easy to figure out, connecting a smoke machine to the proper ports in the Evaporative Emissions system then look for leaks, the Evaporative system runs from the fuel tank to the engine compartment stopping at the purge canister, vent solenoid and other components as well.  If no leaks are found there are other possibilities such as electronics not turning on or off as they should.  This could be anything from a failed solenoid, faulty wiring or problem with the vehicles computer. 

P0420 Catalyst efficiency below threshold.  My favorite, Here is why the code is set.  Starting in 1996 every vehicle for sale in the U.S. had to have a way of monitoring Catalyst function.  The job of the Catalytic Converter is to take Carbon Monoxide (a by product of internal combustion) and convert some of it back to Oxygen, this is done with three precious metals, temperature & fuel mixture.  There is an Oxygen Sensor, or O2 sensor mounted in front of the Catalyst and one after.  The ECM (Engine Control Module) uses data from the O2 sensors to know if the Catalyst is doing its job of creating Oxygen.  Now the ECM does not constantly monitor the Catalyst system, this starts to get a bit complicated but depending on how the vehicle is being driven it is not always possible for the Catalyst to function properly as such in the ECM’s programming it will only monitor the Catalyst function after certain events have been met in a drive cycle.  Once the ECM is looking at Catalyst it uses the voltage values created by the front and rear O2 sensors to determine if the Catalyst is working with in the programmed threshold.  there are a couple of ways to properly test a Catalyst, but the most accurate way is to do a Catalytic Converter efficiency test.  This test involves running the vehicle until it is up to temperature, then install a 4 or 5 gas analyzer probe into the tail pipe of the exhaust, then shutting the vehicle off, disabling spark and fuel (to keep the vehicle from starting), injecting propane into the intake manifold, and cranking the vehicle over and obtaining the readings from the gas analyzer.  The reason for the test procedure is to remove the ECM, Mixture and O2 sensors from the loop, to purely test the Catalyst’s ability to function and it is a pass or fail test with a few Grey areas.  It is not possible to simulate this test with a generic scan tool in the Auto parts store parking lot.  A failed Catalyst is just one of many possible causes of this code and at the time of writing this article the list price for a one piece Catalytic Converter (found in most 1999 to 2004 models) from Subaru was $974.00 not including labor or gaskets.  If the Catalyst is not the cause which is very possible and the culprit is a O2 sensor there are other possible ramifications of not repairing the vehicle properly. If the Catalyst has failed, reasons as to why need to be investigated to avoid a quick repeat of the failure.  The number one cause we see at the shop is a failed head gasket allowing coolant into the exhaust and degrading the Catalytic Converter and or 02 sensors.  But we have also seen contaminated  gas, slow O2 sensors, and Mixture problems cause this as well.  You have to remember the Catalyst function is measured by the ECM as seen through the “eyes” of the O2 sensors.  If you live in Washington State and fail an Emissions test as a result of a check engine light and a PO420 code set, you can request they perform a tail pipe test and if it passes so should your car.  Why is this?  There is chatter that there is a disconnect in some cases between oxygen content in the exhaust and cars that will still run below the legal “measured” tail pipe Emissions standards for that year. 

In Washington state there is a limit an owner has to pay to obtain a waiver in lieu of making a vehicle pass the test and the vehicle is tested every 2 years.  The repair waiver is is granted as long as repairs and or diagnoses are done in the attempt of making the vehicle pass and this work is done by an authorized Washington state department of ecology emissions specialist.  This only applies to unmodified vehicles and is up to the state to decide if you qualify.  Other states have similar programs, and you should check the department of ecology website for the state you reside in for current information. 

P0301, P0302, P0303, P0304, Cylinders 1 through 4 misfire also usually a flashing check engine light will occur as well.  On a second generation Subaru Engine the most common cause would be spark plug tube seals that have allowed oil onto the spark plug wires and plugs, but we also see a fair amount of failed coils, plug wires, spark plugs and some models with an injector failure.  From there a burnt valve, broken ring, rodent chewed wiring and gasket leaks are all examples of things that can cause a misfire code as well.  A flashing check engine light really means you should not drive the car as damage can occur to the Catalyst, rod bearings, head gaskets and the list goes on. 

So one day the light comes on and the gas cap is found loose, but the next time the light comes on tightening the gas cap doesn’t fix it.  Frustrated vehicle owners take cars in to have the check engine light  repaired and it is, for a couple of months but then comes on again.  The owner than takes the car back to the shop where the service advisor, owner  or Technician has the task of trying to explain that most likely its just a new problem.  This is a very common situation that happens and there are a wide variety of reasons as to why this happens.  This is one of the single biggest reasons you should have a place you take your car to for all your service needs.  Taking it to shop A and then to shop B three months later will typically not yield the best results.  If you have lost confidence in a shop on a repeated check engine light situation that is understandable.  A good repair shop will document what codes are set each time the car comes in what was done to diagnose and or repair the condition if it is a different code each time it is most likely a different issue each time.  If there is an ongoing issue keeping one shop involved is a better way to go in most cases.   There are always exceptions to this and only works if the shop you are taking your car too is familiar with the brand.  Sometimes it takes a significant amount of time to diagnose certain conditions that may trigger a check engine light.  A general repair shop is going to have a disadvantage in some cases over a specialist in one make of vehicle.  As we specialize in Subaru we have a huge advantage over the shop down the street that works on every type car that comes in.  Dealership service departments are all about profit and productivity and are generally a bad choice for a problem child that will take a lot of time to get to the bottom of, its just not what they are good at. 

Auto repair is not an exact science, sometimes a good diagnoses is a process of elimination and can be very time consuming as well as frustrating for both driver and shop alike, but it doesn’t have to be that way, a better understanding about how the systems actually work can go along way to clearing up confusion.