I do have the stock exhaust system but no check engine light came on. I’ll work on the o2 sensorAlso think about cleaning and inspecting the O2 sensor wiring and the sensor itself if you can easily remove it, if it has the stock exhaust system and clear codes if they made the check engine light come on. Might be worth removing power to the system to make the ECU relearn how to make the motor work properly once you get the bad fuel out.
How did you clean your fuel filter??i had bad idle speed...thats under control now. i got the fuel filter cleaned (it was very dirty, i need to replace it when i go to town), i didnt touch the throttle body or the plugs. the engine is not as sluggish as before but when i start to drive from a parked positing it struggles intially then as the speed increases it becomes manageable. its not driving at 100% capacity