I have replaced all four O2 Sensors on my 96 Cadillac Deville. They all now report no activity (PO134, PO140, PO146, PO154) and the system stays in open loop mode. I have even tried replacing a pair of O2 sensors with another brand with same results, which I've learned was unnecessary. After testing the voltage on the sensors, I find them to appear perfectly operational ranging between nearly zero an 1.000 volts. Bouncing around , but staying mostly in the .700 area during idle (which I think reflects the slightly rich settings open loop should provide). I have a scan tool that monitors PCM sensors and the PCM reports all four O2 sensors at .450v with only the slighest variation in small degrees. So I know the PCM does not registeractual readings from the sensors. I have tested that the O2 Sensor voltage reads accurately past the connector by using a wire tap. My question is: is there anything like a blown fuse or connector (that I might have inadvertantly disconnected when fussing with Bank 1 Sensor 1 very difficult to reach on top manifold between engine and firewall)? Does this indicate that the PCM may have been damaged? I don't think I cross wired anything, but the engine has been run without the O2 sensors connected to the wiring harness. Any help would be greatly appreciated! Thanks, Keith

 

can you tell us WHY you replaced them? that may hold the clue as to what is going on.

i am thinking connectors / harness assembly.

PCM failures are rare, but do happen. also the pulling out of a pin from the PCM connector can be a possibility. i suggest you pull the connector and check it, grease it with di-electric and see what you got.

you did install the O-sensors with the O-sensor grease?
have you rebooted all the computers?

 

Thanks for the reply. Perhaps it was stupid of me to replace them because the old ones weren't generating errors, but they were originals with 132K miles on them. Was doing other work (radiator, alternator, plugs, etc.) and thought it would be good to do the O2 sensors too. I have not disrupted the PCM, but had disconnected (and reconnected) alot of connectors around the firewall to gain access to the Bank 1 Sensor 1 location (things like EGR, Coil leads, etc.). I did use O2 safe anti-seize compound on the sensor threads, and as I said, they appear to be generating correct singnal based on voltage readings before and after the harness connector at the sensor. I have not checked readings at theharness at the PCM. Will I be able to tell which leads are voitage signals from all four O2s? Are there any relays or other fuses that might be involved (both O2 heater fuses are good and proper battery voltage is reaching the sensors at engine warm up).

How do you reboot the computers? I have reset PCM codes from the dash panel many times. It's possible that the power from battery has remained uninterrupted throughout. I am also sure that I have run the engine without all O2 Sensors connected to their respective harness. The weird thing is they all four report no activity on first road test.