check engine light came on for a day and went back out,what might be my problem

Flux capacitor broke?

Hundreds of things can set off the CEL, you need to get the code read. The computer should have it stored. Auto zone will read codes for free.

Need a LOT more info....engine, code, symptoms, etc.

I need to know if there is a code to put into a 1999 Isuzu Rodeo? I just rebuilt the engine and was wondering if there is any codes I have to put into it to make it good to go.

Is there a code you have to put in after you rebuid an engine??