Scott Mills

I have a new 02 chevy HD with the 6.0. I have had the truck into the dealer several times this week trying to fix my o2 sensor. I keep getting a P0157 trouble code. The sensor is not malfunctioning, it will work on the left side, but not the right. Everytime they put a new sensor the code returns so it is not the sensor. The dealer checked the wiring and said that it functioning correctly, I have taken temp readings off of the Cats and they read about 200 in the front and 380 to 320 in the rear which leads me to believe that the Cats are working correctly. I cant find any exaust leaks either. A previous dealer tried to tell me that the problem was caused by my aftermarket exaust and the fact it wasent creating enough back pressure. The exaust is 2 1/4 inch pipes into a duel in duel out Magnaflow out duel pipes. The dealer the truck is at currently cant figure out what is going on and I am out of ideas. Anyone out there have any ideas?

<small>[ September 06, 2002, 09:59 PM: Message edited by: Scott Mills ]</small>

98blackSS

What do atap readings look like. I'm still suspecting wiring though. A high resistance area in the wiring between O2 and ECM would cause a low voltage reading.

Scott Mills

EFIlive shows it reading about 60mv steady where the left one fluxuates normaly. You think that the wire is grounding out somewhere? Maybe I should insist that the dealer replace the wiring.

wrencher

iTrader: (2)

Take your truck to a good gm shop. they will find your problem. dealers are indepently run unfortunetly there are alot of them that are rip-off's or just don't know w.t.f. their doing.There has to be a good chev/gmc shop in your area. I've had a few rare instances of a pcm cause this problem .good luck. b.t.w. blaming your aftermarket exhaust was their lame atempt to save face after their crap I don't know diagnostics!

Scott Mills

The tally so far: 4 o2 sensors, 1 PCM, another on the way tomorrow, about 40 man hours.... and still no solution.

wrencher

iTrader: (2)

You got to be kidding right? Is this the same place that was blaming back pressure? It must be over there head, what is that a diesel shop!! <img border="0" title="" alt="[Razz]" src="gr_tounge.gif" />

Scott Mills

No I refused to take back to the place that said it was backpressure. This is a new service center. They seem dedicated to finding the problem atleast. By the time they are done I think they will have replaced everythig.

jgkinsofla

Sounds like they don't know what they're doing, for sure. There's a short to ground somewhere in the wiring harness to that o2 sensor, and it's shorting out the PCM also (probably). They can keep replacing pcm's til the cows come home - won't do any good until they isolate/fix the short. I'm just taking excerpts from alldata.com - I'll email you the diagrams also.

Circuit Description

The PCM supplies a voltage of about 450 mV between the HO2S high and low signal circuits. The oxygen sensor varies the voltage over a range from about 1,000 mV when the exhaust is rich, down through about 10 mV when the exhaust is lean.

The PCM monitors and stores the heated oxygen sensor (HO2S) voltage information. The PCM evaluates the HO2S voltage samples in order to determine the amount of time the HO2S voltage was out of range. The PCM compares the stored HO2S voltage samples taken within each sample period and determines if majority of the samples are out of the operating range.

The PCM monitors the HO2S voltage for being fixed below a predetermined voltage. If the PCM detects the voltage is below a predetermined voltage, this DTC sets.

*Heated Oxygen Sensor (HO2S) wire: Sensor pigtail may be in contact with the exhaust manifold or exhaust system.
*An oxygen supply inside the HO2S is necessary for proper operation. This supply of oxygen is provided through the HO2S wires. All HO2S wires and connections should be inspected for breaks or contamination.
*Inspect for an intermittent ground in the signal wire between the connector and the sensor.
*Lean injector(s): Perform the Injector Balance Test.
*Fuel contamination: Water near the in-tank fuel pump inlet can be delivered to the injectors. Refer to Alcohol/Contaminants-in-Fuel Diagnosis. See: Fuel Delivery and Air Induction\Fuel\Testing and Inspection The water causes a lean exhaust and can set a DTC.
*Fuel pressure: System will be lean if fuel pressure is low. Refer to Fuel System Diagnosis. See: System Diagnostic Tables\Fuel System Diagnosis
*Exhaust leaks: An exhaust leak near the HO2S can cause a lean condition.
*Vacuum or crankcase leaks can cause a lean condition.
*If the above are OK, the HO2S may be at fault.

Here's an interesting testing tip -

**If the HO2S voltage is fixed well below 200 mV, this indicates a short to battery ground or chassis ground. If the voltage is fixed near or above 200 mV, this indicates a short to a PCM ground.

Scott Mills

Thanks for the help John, I think I will print the diagrams you sent and leave them in the truck when it goes back for service. Maybe they can take a hint.

jgkinsofla

HAHA - good idea! Actually, when I went back to the dealership the second time for my instrument cluster, I took along the TSB on it, and made sure THEY knew that I knew about it. It seemed to get fixed that time.

wrencher

iTrader: (2)

Please post their "final fix".

Scott Mills

I dont know if they will ever get to a final fix. The truck has been at the dealer for a week now, but I have a sneeking suspicion that it still wont be corrected. What do they do if they cant fix it? I wonder what GM's policy is on this?

wrencher

iTrader: (2)

Where I'm at we have got our share of problem cars, but between everybody putting their heads together &/or talking with gm tech line (it's nice to bitch an engineer out) there is not anything I've come across as unrepairable. Their slow but give 'em a chance.

Scott Mills

And servey says..... its an exhaust leak. A small pin size leak on one of top welds. Plug it up and the problem goes away. Two dealers, a week, 2 pcm's, and 4 oxy sensors to discover a pin sized exhaust leak..... just amazing.

akaceril

you wern't going to parkland chevy were you?

Scott Mills

As a matter of fact yes I was.