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2000 Lexus RX engine misfires, mixture rich, sensor broken

A mechanic recently replaced an O2 sensor on my car. Next day after I picked the car up, check engine light started again. This time I took it to an auto parts store, and they saw 12 error codes on the computer. They told me three cylinders were misfiring and the gas mixture was too rich. They did not give me exact error codes but said fuel injectors may be faulty. I have lo-o-ots of questions here: how could the mechanic not notice these problems when he was fixing O2 sensor? Was he correct to fix O2 sensor? Is it possible he introduced all these problems by fixing the O2 sensor? Should I trust this mechanic again? How much would it cost me to fix these problems and how serious/expensive they are?

anyone can help with this?

Not enough information.
What triggered you to get the 02 sensor changed?
Are you having any problems (other than the check engine light) now (does it seem to be misfiring)?
You need to get the exact OBDII five character codes and post them here if you want good help.
Is all of your maintenance up to date?
How many miles on the odometer?

Well, you can help better. 12 codes is a lot - but the actual list of specific codes would be good. Whenever you have them read write down the exact codes and don’t bother with what the parts store guys say.

Have you gone back to or even called this mechanic? Do you know what the original code was that led to an O2 sensor replacement? (There is no code that says an O2 sensor is bad). There is a possibility that the O2 sensor that went in was faulty off the shelf (it happens) or that something happened with the wiring while that work was done. The O2 sensors are important in helping the computer control the air/fuel mix so it could be involved.

But there are way too many other possibilities. I’d take a trip back to the shop. I’d also find out exactly what all of those codes are.

Get the actual codes. They will be in the format “P1234”

Went back to the store… This time the guy says 12 codes he’s seeing are the same two codes repeated over again. The actual codes are P1150 and P1155

Are you saying that these are the two codes that were there when the O2 sensor was actually replaced? And just to clarify, was only one replaced at that time? B/c those two codes are about 2 different sensors (1150 is bank 2 upstream; 1155 is bank 1 upstream).

But they are also about 2 different kinds of problems. The 1150 is a report about the sensor’s readings of the air/fuel mix (basically its job), and the 1155 is about the sensor’s heater circuit. For the 1150 there are quite a few things that can throw off the air/fuel mix and if one of those is going on then the O2 sensor is just working normally and doing its job. However, it could also be that the sensor is bad.

For the 1155, the odds are much better that the sensor itself is bad, but the problem could also be in the sensor’s wiring rather than the sensor itself.

What you really need to do is get it to a shop with someone who specializes in “driveability” problems - a place where someone knows how to put the car on a scanner and check out the O2 sensor operation.

Was the mechanic correct to replace the O2 sensor? Well, which one? Bank 1 or 2? And even then that could be open to opinion. Many people (including me) hate to toss parts onto cars based on guesses. However, in many cases the diagnostic time required to sort out a code like this can easily add up to much more than the cost of an O2 sensor. So some people think that in general it is actually less of a hassle to just toss in the sensor. It often works, and its easier than having to explain to somebody why they had to pay something like 2 hours of diagnostic time. Customers often don’t want to pay for this kind of “time” - probably b/c they think computers do all of the diagnosis now. Well, they don’t. Computers give clues. A smart human still needs to operate the thing, know what tests to run and know how to interpret the results. It is expert time and costs $$.

Well, thanks for the comment. The codes actually show up now, after the sensor has been replaced. I don’t know which bank was replaced, but will ask. By the way, should mention that “tossing in” the sensor cost me 400 bucks. And I’m sure another trip will cost me more. So maybe he could spend more thinking and not “tossing”

I am absolutely with you there. I just don’t let people “toss” at all. I think that you are justified in going back to this place to have a frank talk with this character about what was done and why and why your car is now worse off. You’ll learn a lot more about whether or not you want to use the shop that way than anything else.

There is just a lot that people don’t think about from the other end. My own guess would be one of three things - one is that the person is pretty much incompetent. A second is that something was inadvertently damaged (like a wiring harness) that now triggers new problems. The third is a complete coincidence. What code(s) precipitated putting in the first O2 sensor?

What kind of shop was it? If it was a corporate chain type of place then don’t expect good mechanics. Find a good local, owner-operated place.