Faulty Loose Gas Cap Warning Light
My first recommendation would be to test the car using the flowmeter on the test station to look for leaks with both pressure and vaccum applied. The gas cap may not leak during a vacuum test but may still be leaking under pressure.
He also needs to observe that the FTP sensor (fuel tank pressure sensor) changes with both pressure and vaccum applied. This sensor is what determines if you have a leak. The power control module (PCM) closes the vent valve and opens the purge valve, which starts pulling
a vaccum. Then the PCM closes the purge and looks to see a change in the FTP sensor. If its readings show no change, you’ll get the P0440 code.
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My “check engine” light came on about 2 yrs ago. I took it to a mechanic who stated I have a loose and crappy gas cap. He told me that if it came on again to check the gas cap. Well, now my gas cap won’t tighten and my car is eating up gas like crazy (I lose 1/4 tank of gas while driving 59 miles). I know that the plastic gas cap is causing my problems and it will also throw off the emission..don’t wait as long as I did. If you have a bad gas cap, replace it.