My car has thrown the engine code P1131 twice now (Lack of Upstream Heated Oxygen Sensor Switch - Sensor Indicates Lean - Bank No. 1) i just switched the o2 sensor to the other side to see if the code follows, if it was something other than the sensor, wouldn't both sides be getting the code, as in both sensors throwing a code? It threw the code the first time, when my mom was driving the car on the way home from me having my wisdom teeth extracted, now it just threw the code last night on the way home from my girlfriends house...it took about a week and a half for the code to come back, so it couldnt be anything too major could it? and i havent noticed any drop in my gas milage either. I just dont like see Service Engine Soon

 

You can get a code on one side for the wiring from the sensor to the PCM. had it on my 97 Coupe. Could be a dirty connection or a wire.

IIRC It takes several exhaust system cycles with a fail before the pending codes will set the MIL.

You can actually see the pending code with the scan tool as pending before it repeats enough times to set the MIL to On.