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Message: <blockquote data-quote="INW-Iron-Steel" data-source="post: 24408" data-attributes="member: 11733">I changed the upstream O2 sensors on my 04 Sierra this last week and now I am getting P0135 and P0155 heater performance codes for the corresponding sensors. Two things I think are strange. 1- I didn't have either of these codes before starting. 2- It can take multiple days and many trips for the codes to trip. For example, after I reset the codes over the weekend, it took until I was driving home from work last night for the engine light to come back. Same codes. So 4 days for the PCM to recognize a deficiency, which seems a bit excessive to me. I am leaning towards bad (new) oxygen sensors right now, but I am interested to see what others think. [ATTACH]3579[/ATTACH][ATTACH]3580[/ATTACH]Here are the codes. I should have mentioned I had the battery disconnected when I replaced the sensors, but I will check the fuses today. I also replaced the sensors with Denso's in an attempt to get quality parts. The ones I pulled out were Bosch's, so the downstream sensors are still Bosch.</blockquote>