Heh, you -DID- save me 80 bucks!

The AutoZone guy didn't have the necessary book to look up the code, but a quick google revealed:

P1456 90 Evaporative Emission Control System Leak detected (Fuel Tank System).

Another gas cap false alarm, same as last spring.

Now I know that I should take it into the dealership and have it looked at under warranty. Emissions control systems, even false alarms, are covered by law for I think 5 years or so.

The cap doesn't need to be off, it just has to be loose. Any loss of vacuum will throw a CEL

I'm aware of that, it's even printed on the gas cap. I always tighten the cap securely, more than the "3 clicks" printed on the gas cap. I think that thermal expansion/contraction is making the sensor false-alarm. That is a characteristic of this time of year, cold evenings and hot days.

Usually, if the car can start 3 times successfully without a fault, the CEL will go away.

This does not appear to be the case with my car, as I procrastinated at least a week before posting anything about this.