Fixing a P0138 code on 2015 sierra 3500 HD with 6.0 liter flex motor

willie bee · November 30, 2023

I bought a new to me 2015 Sierra 3500HD DRW with a 6'0 Liter Flex engine with 38,000 miles on it. Engine light went on and when checked came up P0138 code.

Looked it up on google and stated the O2 Sensor Circuit High Voltage ( Bank 1 Sensor 2 ). Results from Google showed either replace O2 sensor or Change out fuel injectors on Bank1. Since truck is not missing, idling differently, no loss of power I had the dealer changed out both the downstream O2 sensors with Walker brand sensors that I supplied when getting my last oil change. Unfortunately this didn't solve my problem. After noticing my gas mileage hasn't been as good as when I got the truck in March, I took it back to dealer,

The GMC dealer after scaning the truck, said it looked like i'm having a problem with the O2 sensor system. After having a day they said I needed a ECM module and a S modular and installed. After getting it back, the following day the same codes come back. Took it back immediately and truck was there for another few days after putting in a new MAF and driving to reset everything again. I think the light went off again and now there searching and want to install original O2 sensors which I saved and will drop off tomorrow because their GMC sensors . Anyone have any suggestions for I can take to dealer. I think it's a fuel injector that is feeding to much gas or is leaking by its O rings on the rail. Any feed back or suggestions would be greatly appreciated

rav3 · November 30, 2023

In my non professional opinion, your dealership is throwing parts at it at your expense. You need to take it to a good experienced mechanic that can read you fuel trims & O2 readings along with AFM readings to determine what is not within spec. Also, IMO, aftermarket O2 sensors cause problems. You should always replace with OE. Again, my opinion only.

willie bee · November 30, 2023

Rav3,

Thanks for your reply and suggestions, I was up to dealer today with old Sensors and a set of new GMC sensors for him (the dealer) to use if need be.

Also, told him that I think after reading numerous articles on causes that I think he should check all the fuel injectors for leaks or being faulty. He stated that both O2 sensors are now sending the codes, So we'll see what he does over the next day or too. Thanks again.