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Discussion Starter · #1 ·
For about a year my car has been throwing a code for "too rich" (P0121 I think) while in reality the engine was too lean (according to the spark plugs). It did it with 2 different pcm's. Last week I got a replacement front O2 sensor and put it in. That's when the trouble started:

Right after the install the car started fine. And we cleared the code. Shut the car off. Later I started the car and went to drive away. The idle just went UP and DOWN and UP and DOWN and eventually stalled. Very much like a vacuum leak.

We checked for a vacuum leak and couldn't find one. Changed back to the old O2 sensor and no change. Then we autotapped. The MAF sensor was reading a fixed number at all rpm's. We reset the pcm and it did the same.

The next day we put in a new MAF and it worked. Then we switched back to the old MAF and it was working again! We switched to the new MAF anyway. Then, a few starts later, it's at it again. We autotapped again. The MAF was fine. This time the IAC was reading too high. We switched it to another one and it worked fine again. Has been for almost a week.

This morning I put the new O2 sensor back in cause it's still throwing the original code. The car fired up and idled fine. I got a few miles down the road and the idle started going UP and DOWN again and she stalled a few times. I shut the car off, walked some errands, and restarted in 20 minutes. It's been fine again ever since. Except the code for "too rich" is still coming on.

WHAT IS GOING ON HERE? One thought was that the wiring harness is going bad. I can't really be losing 3 sensors at once. The car is immaculately taken care of even at 106,000 miles.
 

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What kind of 02 sensor were you using because I have heard that the Bosch 02 sensor sould give you a bad reading on our cars? If you do have the Bosch might as well get one from your local dealer.
 

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Discussion Starter · #3 ·
It is a stock Delco O2 sensor that I used.

The car did it again today. It seems to be happening once every 2 days now.
 

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Discussion Starter · #5 ·
You're the second one to mention water in the connections. This all started the day after I drove through a small piece of Hurricane Ivan. I think there may be a connection.
 

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Also just want to make sure you do not have an air/fuel ratio guage hooked up.


A local member had the same issue you had. It turned out to be a faulty air/fuel gauge which would cause the o2 sensor to read incoorrectly thus making the car run rich and thro the lean po121 code..

ALso what are your ltrims at idle at?
 

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try disconnecting your battery for 20 min.
Turn your key to run but dont start it. let the feul pump prime and wait 10 seconds. than start your can and let it idle for 5 to 7 min. than shut it off, and wait 5min and start your car again. It shoud now run fine.
The reason for this is your not giving your pcm time to learn. The pcm has to relearn the feul curve give it time and stop driving it cause its making the pcm go crazy so to speek. Also if you switched your pcm for one from a different year you need a case learn performed on it. Dont worry if the car idles high and varies its speed when you do this its suspose to its testing to find the right curve. Hope this helps.
 

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Discussion Starter · #9 ·
If the MAF fuse had blown the sensor wouldn't read on autotap. And it wouldn't be an intermittant problem. But I'll check anyway.

I don't have an A/F guage.

Not sure what those Idle readings were. I know the Idle Air Sensor was reading over 120 so we changed that out. Now it reads around 60 which I'm told is right. Still didn't affect the problem.

My new pcm was case learned to the car. I did try resetting it with no change.
 
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