Ford 2008 F-150 Owner's Manual page 361

If the check fuel cap light
"check fuel cap" message comes on,
the fuel filler cap may not be
properly installed. The light or
message can come on after several
driving events after you've refueled your vehicle.
At the next opportunity, safely pull off of the road, remove the fuel filler
cap, align the cap properly and reinstall it. The check fuel cap
light or "check fuel cap" message may not reset immediately; it may
take several driving cycles for the check fuel cap light
cap" message to turn off. A driving cycle consists of an engine start-up
(after four or more hours with the engine off) followed by city and
highway driving.
Continuing to drive with the check fuel cap light
message on may cause the
If you must replace the fuel filler cap, replace it with a fuel filler
cap that is designed for your vehicle. The customer warranty may
be void for any damage to the fuel tank or fuel system if the
correct genuine Ford or Motorcraft fuel filler cap is not used.
The fuel system may be under pressure. If the fuel filler cap is
venting vapor or if you hear a hissing sound, wait until it stops
before completely removing the fuel filler cap. Otherwise, fuel may
spray out and injure you or others.
If you do not use the proper fuel filler cap, excessive vacuum in
the fuel tank may damage the fuel system or cause the fuel cap
to disengage in a collision, which may result in personal injury.
2008 F-150
Owners Guide (post-2002-fmt)
USA
Maintenance and Specifications
or a
light to turn on as well.
or "check fuel
or "check fuel cap"
361