I have a completely stock 2000 Celica GT. The CEL recently came on with error code P0171, which means the front O2 sensor is indicating a lean mixture. Coincindentaly, this happended about two days after I got it back from the dealership to have a brand new transmission and clutch installed (which feels great now...but that's another story).
I cleaned the MAFS, throttle body, and visually inspected any vacuum hoses I could without taking anything apart (Cleaning the MAFS really smoothed the idle and quickened accelerator response). I reset the ECU, but the CEL came back shortly.
I then took it back to the dealer who did the transmission work, who said that it is running lean, but only at idle, and that the O2 sensors and MAFS are fine. They think it's an "Intake
leak", which I assume could include vacuum hoses, Intake
Any suggestions would be great before I give it back to the dealer to inspect for a day or more.