my check engine light keeps coming on and i have no idea why(323 non turbo)
 

I just put a new tranny into it and now the check engine light keeps coming on. i checked the distributer cap to see if it was damp and nothing. it has been sitting a while, about 3-4 months and i dunno if it just needs new gas or what but the thing should be fine because i didnt touch the engine, block or anything. all i did was tranny, took out the airbox and battery too but thats it. any imput or ideas would be much appreciated. thanks.

When the check engine light comes on take the car to advance auto parts and they will run a check for free.

Unsnap the cover of the airbox and flex the black corrugated ducting going to the engine; look for cracks in it. They frequently leak sneak air into the engine (which of course the MAF doesn't see) and trigger a P0171 code (idle fuel trim lean) which will turn on the CEL.