1. Ok, I got a cap at a parts place. Will get one from the dealer and report back.
2. Next, is there a hatch under the back seats of a Mazda 6 as reported by the Protege owner? I've dropped the gas tank out of cars before but I was 30 years younger and a lot dumber too.
3. No smell of gas at any time even on really hot days.
Thanks.

 

Yes, there is an access panel under the rear seat. But it for the pump, not the vapor canister & it's related parts.

 

Well, it is not the gas cap. A new cap from the dealer was put on Friday and on Sunday the light came on again but after about 5-7 trips. Inspection of the gas cap area shows that paint is worn away around the top of the rim, but when I took the cap off there was a sudden woff of gas smell, suggesting to me that there really was some pressure in there (not the great blasts we got from cars of years ago of course). A real gas pressure loss would be consistent not every once in awhile.
A real electrical short would also be consistent.
A real sensor failure would also be consistent.
What could happen just once in awhile? I do read that the sensor reads amperage at two times and decides from those measurements if the pumping is done. The use of the door lock mechanisms at those times can cause the system to think there is a failure (and of course we always either use the remote or the lock switch on the door). Any truth to that? Thanks much for your assistance!

 

After noticing wear on the gas fill nozzle I put a little bicycle bearing lub in the area where the gas cap o-ring sits. Took a long trip to Canada 1800 miles and many gas fills. The last gas fill overflowed a little, washing away the lub and guess what. We drove home and the next time the car was used the CEL came on again.

 

Did a lot of experimentation with a small CEL reader. I could cause the P2404 error code to be set by after driving some, turn the engine off and quickly open the gas cap. If I delayed that for a minute or two the code would not be set.
Took it to a dealer. They ignored this story I tried to tell them and at first tried to replace the computer (PCU?) on warranty. That of course, did not work and they were somewhat embarrassed since they had told me all this would be free. After a day of testing replaced the evap pump and charged me only the $150 for the part and not the labor. I'm happy assuming this is really fixed.