Each situation in dealing with multiple codes is different. The situation based on data analysis, inspection of components and known common problems are all things tools that should be used when evaluating a car.

Sometimes swapping in a known good component is the only way to prove or disprove a working theory if the testing doesn’t not yield a automatic go/no go type of a scenario.

P0420 can be very difficult for some to diagnose without a Subaru Select Monitor III, most general repair shops would not spend the money on a brand specific OE level tool. Not having the right device may make it more difficult to diagnose than if you did. There are convertor tests that can be done as well, but the select monitor gives you the same type data the ECM is seeing to set the code and light, this should be done in correlation with other tests.

There is an order but there is no “Rule”. Most see a P0420 and automatically assume catalyst and 02 sensors, we have actually seen cars that have had the cats and 02 sensors replaced at a dealership or general repair shop and still set the code, as there was never an actual diagnoses done. I have yet to see a code scanner that can tell me that a mouse ate partially through a intake manifold harness under the intake manifold causing intermittent voltage issues with sensor data back to the ECM.

The market place in many cases does not allow for proper diagnoses. One code is a simple diagnoses the next is complicated, auto parts stores offer code scans and as a result that devalues what a shop offers and we now have drivers that don’t want to pay for a diagnoses, how much time does a shop put into a free diagnoses? Even worse is that not every diagnoses is Conclusive!

Our shop charges for a diagnoses based on what we went through to get results, and as such we have drivers that don’t bring their car to us. We typically try to waive the diagnoses when making repairs even though it hurts when we do. We don’t suggest to replace parts that we can prove as failed typically, but I am aware that other shops do, again mostly based on what the market place has created.

Is this any one persons fault? No but its the state of the industry. There are pockets of shops that will buck the trend, you may not have your car at a shop like that and you may have a car with a situation lending itself to a much more difficult diagnoses.

Thats really about all of the light I can try to shed on your Subaru and is codes.

Justin

Betsy

Why would you replace a car because of the repair bill?

You can give this a read here to address the testing of the P0420 http://allwheeldriveauto.com/subaru-check-engine-light-and-code-p0420-explained/

I cant say from here what your car actually needs however, but I would really hat to see you spend $30,000.00 to avoid spending $1500.00 to $2000.00.

Hope that helps

Justin

Justin

Thanks for sharing what you know with us. I haven’t yet made my way through all 500 comments, but I have a quick question for you. I’ve noticed that the CEL on my 2000 Impreza won’t illuminate at all, even when the ignition is in the “on” position. What do you think?

Thanks,

Joseph

With out a code number there is little I can offer.

The blinking check engine light means STOP DRIVING IT until its diagnosed and repaired. Please consult your owners manual for any clarification about this.

The code will remain in memory for a bit after occurrence so yes a code scan is a fantastic idea and the next step

Hope that helps

Justin

You really need to have someone evaluate the Exhaust valve guides.

Justin