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Runs lean (P0171) but high HC and CO

Greetings. Time for registration renewal and emissions repairs!

Situation: 2001 Prizm (Corolla w Chevy badge).

HC and CO emmissions are just under the wire (NO is average) but the car fails the smog check because an error code is set: P0171, lean bank 1. But engine runs smooth at idle and accelerates about as well as you can expect.

Cleaned MAF; sprayed starter fluid around air system from filter to intake manifold with no sign of engine speed increasing to indicate a vacuum leak. Also did oil change and ran some injector cleaner through the gas tank.

So the HO and CO suggest the engine is running rich. The fuel trim levels, read out by an OBDII tester, are in the +33-40 region for both short and long term.

Some material found trolling the web suggests O2 sensors. Given the fun of replacing them and the fun of paying for them, is there a reliable way to test them? And which O2 sensor should I replace? Front, Rear, both? The car has 120,000 miles, by the way.

Have you tried to clear the code first, and see if it returns? A lot of times, there are quirky reasons why the ECM will read a series of bad readings, and set a trouble code. Sometimes, simply resetting the codes, and letting the computer re-establish the emissions readings will correct things. A persistent code (that returns within a week) really needs attention.

And, if you were to simply throw parts at it, I’d replace the first O2 sensor. The ECM uses this one for fuel trim. The second one, behind the catalytic converter, simply reads the emissions from the converter to make sure it is working right.

Google “fuel trim diagnostics” for some good reads.

Yes, I’ve cleared the code after each step I’ve taken in attempting repairs. The code returns in 5 miles or so.

I like the 1st OC sensor suggestion, but it’s a real bear to get to!


A useful suggestion - thanks!