The check engine light comes on after getting gas and goes off after the tank goes below at least 3/4 of a tank. I changed the gas cap and the light came back on. Today, I was checking the gas cap, took it off in the Jag parking lot and the car locked itself. After retrieving the extra key, I opened it and forgot to put the gas cap back on and the check engine light was off. I put the gas cap back on and the light stayed off. Any clues? A few months ago the car stopped in the desert and the after being towed 30 miles, the mechanic found the mass air flow sensor almost completely plugged. It was cleaned, light was never on, then got home and the next day the check engine light was on.

 

I'm in the UK so the lights may be different but the MIL (SAE official name) is for Emissions only. If the MAF wasn't causing bad emissions the MIL should be off, despite the fault.

To know why it's on you need to read the codes. Many, many threads about that

Mostly the MIL is on after the same problem occurs on 2 consecutive drive cycles (again, lots of threads).