Does the engine light on the dash come on whenever it reads a code on the ecu? or is the light there for main things like oil pressure, temp, etc?

 

Naw, ECU can be holding a trouble code in it without the "check engine" light going on (at least, in my car it didn't go on).

 

Most of the time the computer turns on the CEL when it has a code. Certain codes do not turn on the CEL. If the bulb is burnt out, then it will of course not turn on, even if there is a code. Low oil pressure, high coolant temperature, low brake fluid level/parking brake and battery each have their own lights.