My 09 Focus had a specific warning light that came on first for a loose gap before triggering the CEL. Our 05 Odyssey also threw up a warning down on the dash about the gas cap before triggering the CEL (the Ford I found out by reading the owners manual 
the Odyssey was my wife’s fault)
In this catagory people can not figure out what check charging system means.
Rip off dealer price for a gas cap? A joke?