I started talking about this a bit in another thread related to an old problem but wanted to start fresh here.

The wires/harness going to my front passenger O2 sensor had been partially melted, which exposed wire and had a lot of corrosion. We spliced/soldered in new wire and I started throwing codes like P1153, P0153, and P0155.

Thinking either the splice job was bad or the sensor had gone bad, I got a new one and replaced it. Before we replaced it, we did some logging with EFI Live, which confirmed the sensor was responding slowly. The other was oscillating just fine. Rear sensors are turned off since I have no cats.

So we replaced the sensor and the harness this time. Went back out and logged the car and the sensor still showed the same lazy behavior. In fact, the drivers side sensor now also showed lazy behavior. All we did was unplug it to compare wiring and plug it right back up. The next day, the drivers sensor showed normal activity, so I don't know what the deal was there.. but the one we replaced was still lazy. It basically hangs around 420-475 mV all the time.

So, the car drove fine, mileage seemed fine, nothing wrong, no codes at first. After about 15 ignition cycles I finally get a code, P0154. So that's where it sits. I have driven 150+ miles since first seeing the code, the car still runs great on the road and logs great (no knock or anything) and my mileage is still normal.

My question is, what the heck could be wrong? The fact that the car runs fine and doesn't eat gas makes me want to say screw it but knowing something is still there drives me nuts. Any advice?

 

It's probably the wire splicing that is your problem. All O2s have to vented to outside air. Older 02s actually had a vent hole. The newer ones vent through the wire itself, using the small gaps in between the wire and the sheath.

 

Hmm.. that's interesting. How the heck could you ever "join" wires to get it right then? I talked to the dealership and they said you either have to splice in a new harness (like we did) or replace the main engine wiring harness, which they quoted at $1728. Yeah right.

The main thing I don't get, not that I'm complaining, but how the heck is the car running so well? Is the computer programmed in such a way that if one sensor is whacky then it just runs completely off the readings from the other sensor?

 

Not really, the computer sends out a voltage (450 mv) to the sensor and counts the number of times that the sensor voltage crosses that voltage and whether it is higher or lower than 450mv. 02 sensors use a 0-1 volt scale, so 450mv is the halfway point. It could be reading this voltage which is pretty close to 14.7 afr, making it run decent or it could be defaulting into open loop and ignoring the sensors, which uses the MAP sensor for readings, but regardless, it should still be throwing a code. You can google 02 sensor and there are lots of good articles out there that can probably explain it better than I can.

EDIT: I read your original post again and see it is coding for insufficient activity.

 

Here is some good light reading.


http://www.therangerstation.com/tech.../O2Sensors.htm

http://www.engine-light-help.com/oxy...sor-codes.html

http://www.gnttype.org/techarea/ecms...O2sensors.html

 

You know there is a minimal possibility the 02 sensor you got is just straight CRAP. Also the chance that the sensor is just slower then what the PCM thinks if fine, could be another problem. I have seen on many cars, that anything other then a dealer ordered 02 sensor, just doesn't work.

Get an OEM replacement sensor and try again.

 

I hate regular o2 sensors that is why I always run my car in open loop with a wide band.

 

Swap the front 2 sensors. If the problem moves, it is the sensor. If it stays on the same side, I would look at the wiring for that sensor.

Reb

 

Thanks for the advice and the articles. We logged the car with EFI Live and definitely noticed that sensor is lazy. The driver's side had a very active graph where the other was nearly flat-lined, though it did move a little between, say 420-475 mV. By the way, the sensor I used was an NTK sensor from Summit racing. It was not a factory sensor, though it seemed like people have been successful with basically any sensor.

I also originally wondered if the car was stuck in open loop but if you read about code P0154, I believe it says one condition for showing that code is the engine running in closed loop. This does seem kind of contradictory, especially after reading one of the articles above.. hence I asked the question about the engine running off just the good sensor and make all A/F decisions from there.

Camaro SS, maybe you have a point. Maybe the tune on my car is very good and it's able to run well in open loop if it is, indeed, being stuck there with that sensor giving faulty readings and throwing codes.

 

One thing that could help would be if we knew what each FTC# ment. I know there are difference FTC numbers for open and closed loop, I just don't know what each FTC# means.

Swapping front O2 sensors would be worth a try.

 

Quite correct. You could swap them side to side, and SEE if the problem changes banks. IF it does, you definatly have a bad 02 sensor. If it doesnt change at all... it could be a wiring or PCM issue.