I have a 2003 Toyota Corolla, 140K miles. Back in November my check engine light came on, the mechanic said it was a catlytic converter code, and replaced my catalytic converter. 2 weeks later the check engine light came on again, and the mechanic said my spark plugs were bad, so replaced the spark plugs and got me another catalytic converter. The following week the light came on again, and they replaced my oxygen sensor. The next day (today) the light came on again. My mechanic is stumped. He has no idea why this is happening because he’s checked out everything that could cause this. Any advice?
With today’s complex emissions controls, sometimes local mechanics are in way over their heads. A trip to the dealerships service department may be your only solution. Let them know, right up front, you are not going to pay them to guess what is wrong…You want it fixed and you want a guarantee on their work.
Try another mechanic.
the signals from both the upstream (pre-converter) O2 sensor and the downstream (post-converter) O2 sensor can be read on a scope and the actual signals analyzed.
The upstream sends the signal to the computer to tweak the fuel mix (or tell the computer something is out of the normal range so the CEL can be triggered). The downstream sends a signal to the computer for comparison to determine if the converter is doing its job. A defective downstream sensor can throw a code that indicates a bad converter when the converter itself is okay. Basically the computer is respnding to a bogus signal.
Bad spark plugs would have caused a misfire, which should throw the upstream signal out of the normal range and trigger a CEL. However, I’m puzzled as to why he’d again change the converter.
You need someone to do some more in depth diagnostic work. Someone who can use a scope effectively.