Hey y'all,

I'm not much of a car guy and I am wondering how serious this is likely to be. My car seems to be running the same as ever, but with a little research I've found that it could be anything from a leak to a faulty catalytic converter.

I'd love help trying to diagnose Charlotte and maybe getting a ballpark on what a fair price to get it fixed would be, so I can avoid getting bent over at the mechanic. Thanks in advance.

Pretty much one of the most common codes thrown.

My guess is Master Air Flow Sensor or O2 sensor. I'd clean the MAF sensor (buy spray at O'Reilly, follow instructions, super easy). If that doesn't fix it, probably O2 sensor.

Also check for vacuum leaks. My wife's 2010 Corolla threw that code and it just tried out to be a loose vacuum line. A mechanic had just worked on it and must bumped something lose.

The two most common issues causing the P0171 for that generation of the Corolla are a leaking intake manifold and the rear o2 sensor.

Had a fuel system code come up on a civic because of a gas cap problem once. Pulling the cap off and start it up to let it run (stationary) for a bit with no cap may help too.

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The two most common issues causing the P0171 for that generation of the Corolla are a leaking intake manifold and the rear o2 sensor.

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Just to make sure I understand: Is the rear o2 sensor the same as the downstream o2 sensor?

Yes

Thanks. And thanks to everyone who replied! I'm going to start by cleaning the MAF sensor, then look into getting the o2 sensor(s) replaced if that doesn't help.

If neither action fixes the issue, I may have to take it to a mechanic, because I'll be getting out of my depth at that point.

If you're a cigar guy...

Lean codes are almost always a vacuum leak. Before you buy a sensor, use a scan tool (you can rent one free at AutoZone etc.) to look at long-term & short-term fuel trims and the O2 sensor voltages. Positive fuel trim means it's adding fuel to get the mixture right, probably due to a leak downstream of MAF sensor (i.e. unmetered air). If the trim is fairly high at idle but drops at e.g. 2000 RPM, it's probably a small vacuum leak, since at higher speeds the leak is a smaller proportion of the total air going into the engine.

Once it's warmed up, the upstream O2 sensor should be cycling high/low frequently. I'd have to research what the downstream one is supposed to read.

Hey, here's a video about a 2007 Corolla with the same code:

So, I have yet to do anything yet, but when I drove home the check engine light was no longer on. I am thoroughly confused.

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Charlotte

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Really?

Should I assume that the problem took care of itself, or continue pursuing ways to fix it?