I've attempted to get my check engine light fixed a couple times and at each repair facility they've scanned the code and told me it was a bad oxygen sensor. I had the oxygen sensor replaced at both shops and neither time did the check engine light stay off; it immediatly came back on. Both businesses seemed dumbfounded as to why they couldn't fix my car's problem.

Doing a little more research, I found the following tidbit:

The problem is the an OBD2 scanner will read the code that the PCM is "throwing", meaning that there is a problem in the circuit that contains the O2 sensor. This can be caused by several different parts of that system, EGR valve, spark plug, coils, MAF sensor (Mass Air Flow), etc. There is no code that tells exactly which part needs to be replaced.

So if that's the case, then how exactly do repair shops go about fixing a check engine light for an O2 sensor? You'd think they'd be up to the task since that's their job and yet somehow I've paid for 2 repair jobs that didn't fix the problem and am reluctant to attempt it a third time since its likely to fail.

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    Can you specify the exact code you had? I think it's really necessary for answering this question. I had P0138 some years ago and yes, indeed, replacing the Bank1 Sensor2 oxygen sensor did fix the problem. – juhist Jul 28 '18 at 3:59
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    Please include the year make model and engine size of your car as well as any notes the tech left on the RO. Your tidbit is correct for the most part. On Toyotas for example you can throw codes for the afr sensor while it’s really the post catalyst sensor thats acting up or the other way round. Its pretty easy to spot in scan data. Other cars like Nissans can set codes for an afr sensor when it’s the injectors acting up or pin hole leaks in the catalytic convertor. Or it can be as straight forwards as juhists example. – Ben Jul 28 '18 at 4:34
  • why did you pay two different shops to replace the same part? The first one should have replaced it a 2nd time under warranty. It COULD be a wiring harness. The other parts you listed have their own codes, but I came across a similar issue in a van not too long ago. It was claiming overreading on the oxygen sensor but it in fact was reading correctly but the van was pumping waaaay too much gas into the cylinders due to an intermittent short between two wires in the harness that ran the injectors. – John Lord Jul 29 '18 at 4:37

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