Catalyst Codes and Mechanic's Expensive Diagnosis

The Check engine light came on about a year ago, on my 96 Subaru Outback 2.2l a mechanic said it was the rear O2 sensor and that it was probably just the sensor, which can act up. I decided not to spend the $200, to fix it. I brought my car to a mechanic I had never used before to fix an unrelated problem, he said the codes said both front and rear 02 sensor, and that they needed to be replaced, and that the system should be flushed (Intake, injectors and catalytic system). I reluctantly agreed. Two weeks, car was fine, now the engine light is on again, reading the same code as before the work was done (P0420) catalysic effiency bank.

Mechanic spent about $500 fixing the two o2 sensors now says it will be $630 to replace both cats (front and rear). He did agree to knock off $65 for the time it spent replacing the 02sensors. This seems like a very expensive way to diagnose a problem. It seems like the o2 sensors were fine in the first place. Is this standard protocol? Should the mechanic be held accountable in any way?

I have found a cheaper mechanic who will install parts I find online, I found a company Catco which sells direct fit subaru cats, for half price, anyone have any experience with them?

If the cats are bad its a very safe assumption that the O2 sensor(s) where junk since the cat converter was not doing its job especially the downstream one. The likely culprit was the upstream O2 sensor which can lead to a ruined cat converter and downstream damaged o2 sensor too. Ignoring check engine lights with regards to this portion of the vehicle can lead to pricey fixes.

Good luck.

The better scan tools can graph the voltage signals from the oxygen sensors. A Fluke brand graphing electrical multimeter can, also. The mechanic can look at the oxygen sensors graphs to determine the health of the sensors, and of the catalytic converters. I don’t think they did.
If the catalytic converter(s) and oxygen(s) are bad, the engine could have made them bad. The engine can make new units bad, in a short while.
It can do this if the engine is burning oil, or anti-freeze, or silicone (from sealant), or (and) is out of tune.

I think the money spent on sensors was OK. Andrew made a good point.

i am assuming that you are correct in your timeline you laid out. if so, then you are lucky the car is still running after having the CEL ON for a year.

how did you know that it wasn’t something else, and continued ignoring the problem wouldn’t kill the whle engine?

i doubt the O2 sensors were “fine.” there is an underlying problem which has killed a total of four O2 sensors (and likely your Catalytic convertor as well.)

Your statement that, “it was fine for two weeks after the O2 sensors were replaced”, tells me something. You? It tells me that the engine messed up two new O2 sensors in two weeks! Fix the engine and the problems with the O2 sensors and the catalytic converter will clear up.

Thanks for the information. I don’t think that we can determine that the engine made the O2 sensors bad in two weeks… more likely that there cat is still having a problem. And for a 96+ subaru having the CEL on is a normal occurance. Half of the people I know who drive subaurs have it on and their mechanics often say that it is fine. The comupters and systems are quarky.
My concern now is that it isn’t the cats that are bad, that it is something else. A friend had the same problem, and diagnosis, had the o2 and the cats done and CEL still on… $1000+ later. Hard to know.

You’re saying that, "It’s not the engine that is making the O2 sensors and the catalytic converter go bad; but, that it’s something else. What else is there?

A code P0420 may mean that one or more of the following has happened:

* Leaded fuel was used where unleaded was called for
* An oxygen sensor is not reading (functioning) properly
* The engine coolant temperature sensor is not working properly
* Damaged exhaust manifold / catalytic converter / exhaust pipe
* Retarded spark timing

I have also heard of other things seemingly unrelated causing this code to occur. Knock sensor or bad wiring on the distributor.

i think you’re missing hellokit’s point.

of course one of those things may be causing the problem.

BUT, replacing the sensors and the cat is NOT remedying the problem.

usually the cat doesn’t just ‘go bad’ something causes it.

the sensors have been replaced, and have ‘gone bad’ two weeks later.

doesn’t that tell you something?

the problem lies elsewhere, the O2 sensors and cat problem are just the result of the other (as yet undiagnosed) problem.

I don’t argue that it could be something else, but there is no way to assume that in two weeks the o2 sensor went bad. it could have been that the 02 sensors were bad, as well as the cats, and that the code is reading appropraately now, that the cats are bad. Sometimes I would reset the computer and the light would be off for two or more weeks, then back on again. There is no evidance that the o2 sensors are now bad. This is the tricky thing, we don’t know if replaceing the cats and sensors is remedying the problem or not. Subaru cats and o2 sensors go bad frequently, and if you are familiar with subarus you know that frequently the CEL is on because there are lots of little glitches with the system for whatever reason. I want to know if there is an easy way to check the cats to see if they are working. From the sounds of it, this mechanic said that it is hard to test the o2 sensors because they vary from different driving conditions, so you may check them and they will be fine, but at certain conditions they will not work as well. I don’t think the mechanic is trying to pull one over on me, but rather he just plsys by the book and doesn’t think outside the box at other possibilities.

Somebody has, only a code reading scan tool; and/or, is only using it to read DTCs (Diagnostic Trouble Codes). A scan tool, which has a sizable screen and graphing capability, can be used to see how the oxygen sensors (any sensors) are operating, under idle or under driving conditions.
The DTC P0420:

  • You know if you used leaded fuel. (Can one even find leaded fuel?)
  • One can see, with a scan tool, if the oxygen sensors are operating properly (during idle, or actual driving).
  • The scan tool can be used to determine if the engine coolant temperature sensor is operating properly.
  • Eyes and ears, and scan tool, can determine if there is damage to the exhaust or cat.
  • A timing light, vacuum gauge, and scan tool can determine if timing is retarded, or advanced.
    The scan tool can be a useful diagnostic tool to see how sensors and actuators are behaving (in real-time), when properly used by a knowledgeable person. You don’t have to just rely on stored trouble codes to indicate that there is a generalized problem in a particular circuit (which has a particular component-- such as an oxygen sensor).
    The money already spent on parts and labor could have been saved and used to buy a good scan tool. Now, all you need is a mechanic who knows how to really use one.