Every few years, my CEL will illuminate on the '00 Celica. My old friend, P0171, will display on the code scanner, and I’ll know that it is time to pull the MAF sensor and hose the innards down (gently) with cleaner to remove the oil+dust coating. This sensor, when dirty, causes the vehicle to think that less air is coming out of the air box, and it trims the fuel accordingly. However, the pre + post catalytic sensors say otherwise, flagging a P0171.