check engine light comes on after replacing gas cap

Then you have a leak in the fuel vapor control system. Could be the cap, purge control valve, cracked vapor line, etc.

Did you replace the cap because the light was already on and the new one didn't do the trick ?

The EVAP system is probably the most difficult system to get the computer to run a diagnosis on. For starters the gas gauge must be between one quarter and three quarters. If it's not the computer won't test the EVAP system. Best to clear the code with a code reader or scan tool and see if it comes back. Hope that helps! Jim