Fuel Cap and EVAP system
 

Hello,

I have 2003 focus.

fuel light keeps coming on even when cap is secured. When travelling long distance I quite often get an engine light which when I check gives me vapour leak code. To me this points towards a leaky fuel cap.

How does the sensor on the fuel cap work?
Is it testing for vacuum or is it just detecting the fuel cap?
Does the fuel cap contain a valve?

Inside the filler tube for the fuel tank, the metal flap that covers the tube when no nozzle is inserted doesn't close completely would this affect the sensors operation?

thanks