I recently had my timing belt changed and after they did the job I noticed while i'm in park after the car has warmed up or at a stop light the rpm's bounce from about 700 to 1500 to 400. Sometimes I think it may stall. Did the shop not put something back properly while doing the timing belt job or is it something else. I do know I need an O2 sensor is that the problem.

 

any mods on your car?

 

no mods

 

O2 sensor could certainly play a part. The IAC system in the Lancer is very finnicky, so I wouldn't be surprised at all if that were the culprit. Replace the O2 sensor, make sure to reset the ECU, and go through the reset procedure. When you've gotten a few miles on it for the idle tip in/out to teach itself again I'm willing to bet your problem will be fixed. BTW. AT or MT?