A loose fuel cap is one of the most common reasons why the check engine light turns on.
Check engine light doesnt come on with key on.
It then goes to the electronic control module.
The light bulb for cel has a power feed with 12 volts runing to it with key on all the times.
Since you said the other lights work then.
I doubt any of the things mentioned have any affect on the check engine light.
It notably prevents gasoline fumes from leaving.
Radio on and full systems on.
This way you can sit and listen to the radio without everything else draining.
Do not check resistances.
Radio on mode only allows you to use the radio.
If the check engine light doesn t come on in the run position it means the computer failed the self test mode and there s a problem with the computer.
This unit is a solid state unit.
The cap is a crucial part of a car s fuel delivery system.
When turning the ignition switch to the run position the check engine light should come on.
There is 2 key modes on the switch.
This is called the self test mode for the computer.
Ok it looks like fuse 7 in the dash fuse box supplies power to the check engine warning light.