I have an 07 Trailblazer LS with 84,000 miles. When I fill up, the check engine light comes on, after 3/4 tank to about 1/4 of a tank of gas the check engine light is off. When it hits 1/4, it goes back on. Also, a lot has to do with humidity and temperature. The only code ever pulled was for a bad gas cap. Ihave replaced the cap once with an Auto Zone cap and twice with OEM caps. I have even rubbed Vasoline around the rubber ring of the gas cap and the filler neck. I know it has to be an emission leak somewhere, but have no idea. It doesn't effect performance or gas mileage. It is just annoying. Any suggestions?
weird check engine light
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Mooseman
Moderator
I think it's your pressure sensor on top of the gas tank. It's located right on the fuel pump/sender assembly.
There is no code that says "gas cap" specifically. It's always a failed leak test of some sort. Could be a crack, loose hose, or bad purge or vent valve. See if you can get the precise code. Edit: Or as Mooseman says, the sensor.