QuestionQUESTION: Approx. 2 years ago I bought a 1998 Forester with 65000 miles. The overflow had green coolant in it with a very slight dark tint to it. Very intermittently the car would get hotter then average, but never overheat. Eventually my Subaru dealer diagnosed the condition as head gaskets. They were replaced along with many other gaskets and seals. Within 1 month of this repair the MIL light returns. It has a P0420 fault in it. The mechanic insists on the catalytic convertor to be replaced. I am not so sure as I have more of a feeling that the front O2 or even the rear O2 would have been affected by the coolant going through the exhaust. I am not in the mood to spend 1400 dollars for an OE equipment catalytic convertor if there is no 100% guarantee that this issue will be repaired. What are your thoughts? Also the light will come on on very hot days with the A/C running after a long drive and sitting idle for 8 min.The light has now extinguished itself, and has not come back on. If a catalytic convertor is malfunctioning, being that it is a mechanical device checked by 2 electronic devices, how can it be "bad" and then good? Either it should fail and stay that way, or it is ok and the sensors should be faulty? If it is the sensors, which one should be changed first? Any help would be appreciated as I plan on keeping the car for a long time, even though it has cost me a lot of money in the first 2 years of ownership.
ANSWER: Ingo,
Ok here is the deal PO420 Catalyst Convertor Efficiency below threshold is a CONDITIONAL code. Not a component code. Meaning the ECM (engine control module) will command on the check engine light whenever the "Condition" of a Catalyst working below the minimum threshold specified by The Federal Government and the manufacturer.
Its not the same thing as a component code which would be like a P0130 02 sensor malfunction.
A conditional code does not point to any particular part to be replaced at all. Instead a proper analyses must be done including a Catalyst efficiency test which is done with propane and a gas analyzer and is also the only test approved by the department of Ecology.
If a cat fails the test it fails the test, but that Does not mean it will Fix the Condition the ECM is seeing, but may have suffered damage as a result of a slow 02 sensor or coolant entering the exhaust from failed head gaskets. A car can have a Catalytic convertor in it that fails a catalyst effiencey test and Never set a check engine light.
The ECM uses the readings from the front and rear 02 sensors to determine if the Catalyst is working as designed. The ECM does not always look at the Catalyst Efficiency as Specific events must occur as programmed to determine if the Catalyst is ok.
You can read more here http://allwheeldriveauto.com/your-subaru-check-engine-light-explained/
You may in fact have an issue with one or both 02 sensors, Or something else. Proper testing will prove this true or not.
So to answer the question as far as how can an intermittent failure with a mechanical device occur? First of all a Catalyst is not a mechanical device it is a chemical one. There is a chemical reaction that happens in a catalyst that converts some of the exhaust gasses back into something less harmful.
There are a lot of factors involved and it is not a black and white situation at all. Nor is Auto repair an exact science in its current form. To truly Diagnose the problem, hours upon hours of extensive testing would have to be done but the public doesn't want to pay for this, instead a quick knowledge based assumption (usually done within an hour) based on the current data is used to come up with a "most likely is wrong".
All of this is as a result of parts stores offering to give customers the "code" for free. When the code is just a starting point. Franchised auto repair devaluing a diagnoses to try and "get you in the door" and the American consumer ultimately driving "over competition" in the market place.
You are right $1400.00 is alot of money and more than my shop charges for the repair with O.E. cats.
What is the price for clean air?
There is no pleasure putting any money in your car ever, unless you are having it washed.
Wait until the 2007 cars are 10 years old, the cost to fix check engine lights will be a whole lot more.
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QUESTION: Well there is no price for clean air as it is priceless. Basic fact is, if we do not have it, we all die. Harsh, but true. The fact also is though, that I have spent thousands to repair this car, and now I will be stuck with another large bill. I have a tough time not going with OE parts, as I realize how much research and development goes into designing proper parts for cars; hence visiting the dealer for repairs instead of an independent. My question was more directed towards the coolant contamination that must have happened when the head-gaskets started to leak. It was not as obvious or sudden as a GM 3.4L engine would have been. It took a long time before the concern became more apparent. However if you feel that it could be a cat or one of the two O2 sensors, or both, or all three things, I am unfortunately not much farther ahead with my decision. On some of the other Subarus that have had the head gasket issue, have you seen more O2 sensors fail or catalytic convertors? Please advise.
Regards,
Ingo
AnswerI wasn't suggesting aftermarket parts at all.
We use Genuine Subaru parts, and just don't charge as much labor.
The same is typically true of most shops that specialize
The 02 sensors can and will fail if they are subject to coolant.
Here is what we would do if the car was here.
Test, test and test.
After testing if we had determined the cat was marginal, (its all about the gas analyzer readings while propane is injected during a disabled fuel and ignition event), and the o2 sensors suspect we would perform an induction service to help clean the carbon off of the catalyst that can affect performance.
We would than replace the suspect 02 sensors. And give it time even if the light came back on once or twice after, we would clear it for our customer at N/C and have them drive it some more.
If after testing, the car falls into this scenario I can tell you we have had great success and saved quite a few customers a lot of money.
I have to tell you the brutal truth. Shops are in it for profit and the service department at a dealer is now how the dealer makes money as the Internet has made it impossible for the new car dealer to make money on sales of new cars and now the service department keeps the whole thing going. As such quick easy repairs keep production high and costs lower. As a result the Service in Service Department is gone. I think it may be difficult to find a dealer service department that would be willing to do what I have mapped out above.
Justin
