FORD 2006 Expedition v.1 - Manual (Page 218)

The tire pressure should be checked periodically (at least monthly) using
an accurate tire gauge, refer to Inspecting and inflating your tires in
this chapter.
Understanding your Tire Pressure Monitoring System (TPMS)
The Tire Pressure Monitoring System measures pressure in your four
road tires and sends the tire pressure readings to your vehicle every
minute while you are driving and once an hour when your vehicle is
parked. The Low Tire Warning Lamp will turn ON if the tire pressure is
25% below the pressure listed on the Safety Compliance Certification
Label (approximately 6 to 9 psi below the manufacturer’s recommend
tire pressure). If the tire pressure increases 2 psi above the “Light ON”
threshold, then the TPMS light will turn OFF. Once the light is
illuminated, your tires are under inflated and need to be inflated to the
manufacturer’s recommended tire pressure. Even if the light turns ON
and a short time later turns OFF, your tire pressure still needs to be
checked.
In short, once the light has turned ON, at least one tire may be under
inflated.
When your temporary spare tire is installed
When one of your road tires needs to be replaced with the temporary
spare, the TPMS system will continue to identify an issue to remind you
that the damaged road wheel/tire needs to be repaired and put back on
your vehicle. During this time, the low tire warning light can periodically
return or stay on, depending on the state of the damaged road
wheel/tire. This will include the message center messages (if equipped).
To restore the full functionality of the Tire Pressure Monitoring System,
have the damaged road wheel/tire repaired and remounted on your
vehicle. For additional information, refer to Changing tires with TPMS
in this section.
When you believe your system is not operating properly
The main function of the Tire Pressure Monitoring System is to warn you
when your tires need air. It can also warn you in the event the system is
no longer capable of functioning as intended. Please refer to the
following chart for information concerning your Tire Pressure Monitoring
System:
2006 Expedition (exd)
Owners Guide (post-2002-fmt)
USA (fus)
Tires, Wheels and Loading
218