It seems that the gas cap error is just a generic error code given when something goes out of specs, so the sensors flags the ECU for the check engine light! You said, your X40 was on a incline. First was your gas low? This can cause a check engine light to come on, something to do with the amount of fumes & gas in the tank or purge tank. I try and never get under 1/4 tank now :) I hate it when I get the check engine light, dealer said it was gas cap! Yea right. Remember that an X40 needs something like 6+ gallons to start up if run dry. Knowing this tells me that the fuel system is very sensitive too how much gas is in the tank.