i have a 2004 blazer that was getting P0155 fault. I changed Bank 1 sensor 2 this sensor which seemed to clear the problem but after a few weeks, received a P0155 along with a P0141. I changed the bank 2 sensor 1 and now I get a P0135, P0141 and P0155 DTC's all at once. I've verified 12V + heater voltage, grounds, and .4v coming from the PCM on all O2 sensor connectors. I have graphed what I think is normal O2 sensor operation using a diagnostic scanner with nearly 1v during preheat and then normal slow switching during idle and faster switching during 1.5 to 2K rpm testing. If reset the codes and keep driving, the car will pass all readiness tests, except EVAP tho I've only driven it 50 miles and I am unsure how to initiate the EVAP test. Once the car is cold and I restart it will set these 3 above codes. I currently have the stock sensor on Bank 1 sensor 1, and 2 new bosch sensors on both bank 1 sensor 2 and bank 2 sensor 1. The car has had a complete tuneup but has a slightly rough idle. Driving seems normal and has plenty of power. Anyone have an idea here? I'm at a loss for new ideas except perhaps that the PCM / ECU is bad.

 

*Moved to a thread of your own rather than into an old thread that is loosely related*

Have you checked the wiring from the sensors to the PCM?

 

I have checked wiring. It all appears fine. Approx 9 to 10 ohms on the heater circuit for each O2. Yesterday I installed 3 new O2 sensors warranty Bosch replacements. Started the car and had no codes. Ambient temp was 65 deg. This morning I go out, connect up my diag tool and set to record on DTC fault. I turn the key and start the car, and instantly I get P0141 and P0155 error. I've already verified wiring and that I get 12V to each O2 connector. How does GM determine an O2 Heater Circuit Malfunction? Is that done directly in the ECU via the wiring, or does it monitor the O2 sensor output and if it doesn't respond quick enough, set the code? I am beginning to think it's the ECU itself or the Bosch brand O2 sensors.

 

Have you looked up the codes in the DTC Index located in the Tech Article (DIY) section? There are diagnostic charts that will lead you to the actual source of the problem rather than shotgunning it.

And I wouldn't put Bosch O2 sensors in my vehicles if they gave me an infinite free supply of them. AC Delco every time for me.

 

I am checking that section out now. Very descriptive and seems to match up with alot of what I've read in a service manual for a slightly older blazer. My gripe is that the DTC should be set if the O2 sensor voltage remains within 150mv of it's startup voltage for 130 seconds after a cold start. I'm getting 2 O2 sensor faults within 3 seconds of engine start, and proper O2 transitions within 20 seconds of engine start with closed loop operation in about 70 seconds. I just don't get why or how it's testing the O2 sensor in 3 seconds and why it's failing. This is why I suspect the ECU. From what I've read you can't just swap in a new ECU, it has to be programmed. I am going to do further testing today. It just takes forever for complete cool down, with ECT to be within 9%F of IAT. What a pain.

 

If you do end up swapping out the PCM, look into getting one from any one of the online tuners. I had an issue with the programming in my Rainier that was out of warranty. Rather than get the TSB's done at the dealer for over $200, I purchased a tuned PCM from PCMforLess for $150. I received the new PCM with all of the updates, programmed for my truck, and hopped up a bit as well.

But, about your problem. How good did the ground register at the plug for the O2 sensors?

B1S1 grounds out to the back of the driver side cylinder head while B2S1 & B1S2 (or B1S3) ground out to the rear of the passenger cylinder head.

Also, were you getting a strong +12V signal to the heater in each O2 sensor? Have you tried to load this circuit to see if the voltage stays constant (using a lite resistor)?

 

I did a resistor test on the heater circuit. Works fine with no significant voltage drop. Replaced the ECU and that seems to have cleared up 1 code but not both. I'm going to try some AC-DELCO or Denso O2 sensors as there is little else I can think of. I've verified solid reference voltage, ground and heater voltage. Anyone know why the P0141 and P0155 codes would post so quick? < 3 seconds? Does the ECU perform a test and store these codes to be posted next time the car is started? And of course the fun part is, I can start the car for 3 seconds, shut it off. Clear the codes, restart and codes do not appear again, even tho all temperature criteria are correct and the test should be performed again, it does not fail it.

 

Somehow, the PCM is detecting a problem with the circuits on the initial check. This could be an excessive resistance or a possible open circuit detected (regardless of whether it is present or not). If the circuit checks out ok, then the problem could be with the PCM itself. If you have the means to use a known good PCM for a bit, that may be a good step to take.