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Ron Z.
Ron Z., - GMC Tech -
Category: GM
Satisfied Customers: 18775
Experience:  18+yrs experience. State Inspector & GM Diagnostics/Repair
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GMC Yukon XL 2007 GMC Yukon XL 5.3L flex fuel engine with

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2007 GMC Yukon XL 5.3L flex fuel engine with 95k miles. I drove for about 100 miles without the gas cap on and now have an engine light the dealer tells me is due to faulty post-cat oxygen sensors.

Question: Is there a way to clean this, or do I just need to replace? What is the best type of oxygen sensors to get and where should I get them? Should I replace all of them or wait?

Welcome to! 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.


Customer: replied 5 years ago.
The dealer did not provide me with the codes, but told me they indicate bad post cat O2 sensors. I believe I have a code reader that could remove the code to see if it goes away/comes back. I can also call the dealer to see if they have the code documented.
It'd really help a ton if we had that p-code. Like I say, most commonly, Oxygen Sensors fail one at a time. Very rarely BOTH go at the same time. Juist hate to see you spend money on something that is more than likely due to a missing fuel cap! If you can, read the p-code, and let's diagnose this correctly!
Customer: replied 5 years ago.
If and OBD-II will work, I will get the code(s) tonight. I think I can also go to Advance Auto and have them hook up and get the codes. Thanks for your help. We will be in touch.

Right. Most Advance and AutoZone locations read codes for Free...


Customer: replied 5 years ago.

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.


Customer: replied 5 years ago.
#8 which is a 10a fuse was blown. I erased the codes, replaced the fuse and the engine light came back on immediately. I then unplugged both post cat sensors and bagged and taped them up. I erased the codes again and replaced the fuse. So far the fuse hasn't blown. If it continues to hold up I am assuming one of the two sensors has a short. I'm thinking I should just replace both of them. Do you agree with this approach?

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.

Customer: replied 5 years ago.

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.

Yep. Thats going to be the problem. There is going to be a short in one of the wire harness. Unfortunately at this point- it's a matter of visual inspection to find that short. More than likely, a rubbed through wire touching (grounding out) on metal somewhere.
Customer: replied 5 years ago.
Can you provide a schematic of the harness(es) that would be affected so I am not chasing ghosts?

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.



Customer: replied 5 years ago.

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.



Nah. THe O2 sensors are replaced on a need-to basis. Like I said from the git-go, very rarely do both go at the same time. And if the fuse is blowing, I highly doubt it's the sensor at all! I'm leaning more towards a short somewhere. Glad you didn't let the Dealer replace 2 sensors for you now, aren't ya?!?!
Ron Z. and 6 other GM Specialists are ready to help you
Customer: replied 5 years ago.
I want you to know I appreciated your expertise on my issue. I have a busy week and weekend, but hope to get my visual inspection completed by Monday. I hope I can find where the short is, but and concerned it will be in a spot hard for me to get to. If you have a diagram of the wiring harnesses for me that would be great.
I tried looking for one last time you asked, and I couldn't find one. All I had was the schematic. Unfortunately, it's a visual hunt at this point. Maybe even an electrical shop might be a good idea, as they are experienced in finding shorts.
Customer: replied 5 years ago.
Hard to see from the Pics, but sure enough I found the short. I was the harness coming from the passenger side to the driver side. Cable mgt was moved, I think, and allowed the driveshaft to rub up against it, breaking one gray wire and scalping two others. I crimped on some connectors (really well), taped up each wire individually, stuffed everything back in the wire mgt. and taped the whole thing up. I then replaced the #8 fuse and cleared the codes. Everything seems to be working perfectly -- so no need to replace the O2 sensors.

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!!