2009 GMC Acadia P0036, P0140, P0141 Diagnosis: Post-Cat Oxygen Sensor Issues in Qatar

Hi everyone, I'm currently in Qatar (Middle East) and face a significant challenge with local automotive services. The dealership here is the only one available, and there are very few reliable or trustworthy independent mechanics—none that I know of. Your expert input would be highly valuable as I try to understand these issues before deciding on whether to take my vehicle to a repair shop. My 2009 GMC Acadia's Malfunction Indicator Lamp (MIL) turned off recently. After scanning with an ELM327/Torque scanner, I received three diagnostic trouble codes: P0036, P0140, and P0141. From my research, these codes relate to the post-catalyst oxygen sensor—specifically indicating issues with the heater circuit of that sensor. I reset the codes once to see if they would reappear, and they returned immediately. Since then, I haven't driven the vehicle much. Previously, I experienced two failed fuel injectors, which led me to replace all six (I found affordable parts on RockAuto.com and assumed labor costs would be similar at the dealership). This was necessary because the car stalled with significant exhaust smoke—back then, three related codes appeared (I don’t remember them now), all tied to fuel line pressure. At that time, I was concerned about potential damage to the catalytic converter. The dealer confirmed the cat was fine, and I’m now worried this could be a similar issue. Additionally, my air filter is due for replacement—though I noticed it was slightly dirty when I removed it. I don’t believe this directly causes the current codes. Any insights or troubleshooting steps would be greatly appreciated. As a follow-up, I plan to perform the suggested diagnostics on each code over the weekend. P.S. I’ll update with my findings once I complete the testing.