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2009 chevy
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Modern vehicles don't have gas cap sensors.

What they have, is a sensor that checks the pressure inside the tank.
This sensors lets the truck start and run for a defined period of time to allow the tank to pressurize (this takes longer when the tank is near empty).

After the defined time, if the pressure is not at the correct value, the PCM assumes a pressure leak and throws a code.

This normally causes the CEL to light.

I know there is a separate indicator for a loose cap, but I have never seen it light.
 
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