Lean 02 sensor reading

[flyboy1294]

Recently, I was driving down the road and my SES light came on. Seeing it, I was concerned and checked everything out once I got to my destination. Nothing was apparently wrong, so I got some fuel and unhooked my battery cable for a few minutes and cleared the code. On the drive back home, the light never came back on.

Then today, I went to the shop to have my headers tightened up (suppose to have them tightened after the first 100mi), and while I was there I had them check my error code history. It came back to say that my right 02 sensor had been reading lean and setting off my SES light. The mechanic said that moisture in my gas could cause the 02 sensor to read lean. Thinking back, before the trip, I had gotten gas at a place that was notorious for low-quality fuel. Then once I arrived at my destination, I fueled up at an Exxon station. In my mind, this validates his suggestion. In the mean time, I will only get gas at my local BP or Shell station.

Anyway, I was curious if anyone else had any knowledge of what could have caused this reading besides what the mechanic suggested. If there is something wrong, I would like to take care of it now vs later after it does damage.

 

[big_mike]

Well since your a 98' thats got the OBDII with the (2) pre-cat sensors, I'm guessing those are the only ones you kept?

Several things can cause a lean O2 sensor code. I chased that on my Trans Am recently.

Bad gas (as already mentioned)
failing O2 sensor
exhaust leak
partially plugged injector on that bank
loss of fuel pressure

I'd say if you ruled out exhaust leaks, clogged injector, fuel pressure & bad gas it could be a O2 sensor on the way out. I'd just replace them both & see if it ever happens again. If that doesnt fix it then at least you got new sensors & know its time to check elsewhere.

 

[flyboy1294]

Well since your a 98' thats got the OBDII with the (2) pre-cat sensors, I'm guessing those are the only ones you kept?

Several things can cause a lean O2 sensor code. I chased that on my Trans Am recently.

Bad gas (as already mentioned)
failing O2 sensor
exhaust leak
partially plugged injector on that bank
loss of fuel pressure

I'd say if you ruled out exhaust leaks, clogged injector, fuel pressure & bad gas it could be a O2 sensor on the way out. I'd just replace them both & see if it ever happens again. If that doesnt fix it then at least you got new sensors & know its time to check elsewhere.

Thanks! That was very helpful. You were correct, I only kept pre-cat 02s. I would guess it was either bad gas, a dying 02 sensor, or a possible exhaust leak (The headers were new then. I have since had the bolts re-torqued.) out of the causes you listed. I'll watch where I fill up from now on, and make sure those headers are tight. If it comes on again, I'll replace the 02s as you said... they are pretty old if I remember right.

 

[big_mike]

Yea, replacing O2's is so easy & really not expensive that its always a good start. sometimes they will play hell with the PCM when they are dying.

I try to be helpful on here. I'm FAR more educated in LS engines but I'm learning these trucks pretty quick.

 

[flyboy1294]

Well the SES light came back on today. I have been running the more expensive gas, and I know my headers are not leaking. I am taking it back to the shop Monday and checking the code just to be sure, but I am willing to bet that I will be purchasing new 02 sensors this week.

 

[Wwildman]

If the o2 sensor has a lean code the the only other thing it could be related to that would be a vacuum leak after the maf or a MAF or MAP sensor causing issues

 

[big_mike]

If the o2 sensor has a lean code the the only other thing it could be related to that would be a vacuum leak after the maf or a MAF or MAP sensor causing issues

If that bank is having any fuel issues it will also get a lean code.

 

[flyboy1294]

Taking it to the shop right now.

So its either fuel problems, a vacuum leak, or a bad sensor somewhere. This should be interesting lol.

 

[Wwildman]

Does that shop have a smoke machine ?
It will help find the vacuum leak

 

[flyboy1294]

Got it fixed. Turns out the tech told me wrong the first time about the 02 sensor reading. The problem was a bad coolant temp sensor. It was telling the PCM that the truck was either too cold and causing it to run rich, or that it was too hot and causing it to run lean. Replaced the part and the SES light went off.