Welcome to JustAnswer.com! My name is DriveFast71(Ron) and I will do my best to answer all of your questions completely and accurately.
Did you have any "p-codes" read to "make sure" it was oxygen sensor? Driving without the fuel cap on will set a check engine light and a p-code of P0455. Usually 2 Oxygen Sensors do not go faulty at the same time.
Right. Most Advance and AutoZone locations read codes for Free...
The OBDII reader had 4 codes:
I erased them and only the p0036 and 56 came back
Ok. These codes are pointing to a fault with the rear heated oxygen sensors, HOWEVER, they point to a problem with the internal heaters. The chances of both internal heaters going bad at the same time, are slim to none. What I think you have here is a problem with the power to these sensors. In the underhood fuse box, check the fuses, especially #8, #36, #52. In spect the wire harness and electrical connectors to these rear sensors for any loose, broken or rubbed through wires. I'm much more confident that this is a problem due to lack of power to these sensors, rather than the sensors themselves- so save replacing the sensors until the last resort.
Yes. If that fuse is blowing there is a short in one of the sesnors or the wiring to those sensors. Try an inspect the wire harness as far up as you can and make sure there are no rubbed through wires touching metal.. Unfortunately GM i snot providing an Ohms specification, or we could test them and find which one has the short. I hate to see you replace both when probably only one is at fault, however, at this point, it might just be the easiest way.
I may have some bad news -- for me. I disconnected both post cat O2 sensors, bagged and tapped both ends -- and the fuse still blows.
I am thinking this means I have a wiring problem, but need you to tell me if it is possible that is normal if you have no sensors hooked up.
If the only way the fuse can blow, if sensors disconnected, is a short in the harness I suspect it is the passenger side since that harness appears to go under the engine.
Please give me some direction to go from here.
Below is what I have for electrical diagrams. It isn't going to help you much. The best way to do this is to start at the connector that plugs into the sensor, and work your way up. It's not "chasing ghost" it's visualy inspecting the wire harness for rub-throughs or signs of damage. The entire wire harness needs to be checked. Unfortunately, the only way.
Thanks so much for all your help. It looks as if neither sensor may need replaced. But if one did replaced, would you recommend only replacing one? I thought that chances are if one is bad, the other may not be far off.
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AHA! Wish the Dealer's would pay more attention to what is actually going on, instead of replacing parts at random!!
Glad you found the actual cause, and saved a boat-load of money!!