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Toyota 2001 PO171

I have a Chevy Prism (aka Toyota Corolla) 2001. The Check Engine Light came on with code PO171 – engine running lean. My local mechanic, Darryl, replaced one of the two oxygen sensors (I’m not sure which one). The light came on again, with the same code. He says other problems can surface after an original problem is solved.

Chris, at the Toyota dealer, says Darryl should have checked whether the oxygen sensor was broken before replacing it. Chris said this is an easy test.

Darryl, plus another guy at the Toyota dealer, said there is no way to check whether an oxygen sensor is broken, other than replacing it and seeing if the Check Engine Light goes on again.

I?ve researched this on the web and found out there are numerous reasons for a code PO171 ? oxygen sensor, air flow meter, mass air flow sensor, to name just a few.

My question is: Should I take the car back to Darryl, since he did the first repair. Or, should I take it to the dealer, that charges way more, & that has given me inconsistent information? Do these guys just replace the most likely part each time until they solve the problem? That could get very expensive!

Oxygen sensors can be checked with either a voltmeter,a scan tool or a lab scope.

The best tool is the lab scope,you can compare the waveform with examples of what it should look like.

With a voltmeter you look for a cycling voltage signal,but the readings are harder to interput than looking at a waveform on a lab scope.

All this info is in your basic engine performance chapters of your textbooks.

So Daryl shot the messenger eh

From the factory shop manual for my 02 Hyundai Sonata:

code PO171 possible trouble areas:

1 Low fuel pressure

2 Faulty or clogged injector.

3 Leak in intake system.

4 Leak in exhaust system.

5 Faulty MAF sensor.


If an codes relating to injectors O2 sensor, engine coolant temp sensor or MAF sensor are also stored, do all repairs associated with those codes befre proceeding with this trouble area.

If Daryl (or you)had access to the above info he would not have wasted his time & your money on replacing a perfectly good O2 sensor.

Thanks for your reply, oldschool. It appears Darryl just did the most likely repair, without checking that the O2 sensor was the problem. I think if I took it back to him, he’d just go down the list, probably next replace the second sensor. So, I’ll bite the bullet & take it to the dealer & make sure I get Chris to work on my car.