My daughters 2000 concorde has the check eng light on, code p0455 according to auto zone. Evap.... where should I start ??

 

Start with the gas cap.
Also check all vacuum lines near throttle body if it is a 2.7 engine.

 

Gas cap is tight, can it still be bad?

 

its a 3.2l

 

Cap has check valves built in to it, could be bad. Cheap swap, maybe replace it, reset the codes (disco battery for a few minutes) and see if it comes back.
Evap codes take about a week to "mature" meaning a number of drive cycles to set the code.