Why would the check engine light come on after filling the gas tank on a 2000 Isuzu Rodeo?

First, reset the computer by disconnecting the negative battery cable for a couple of minutes then reconnect. If the CHECK ENGINE light still comes on, check the gas cap and make sure it is tight. Due to emission rules (OBDII), the computer checks to make sure the gas cap seals the gas tank. You might have to replace the gas cap.