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The system will reset when the vehicle exceeds 20 mph for a short time. If the warning light satay on past that point the sensors may have to be Pinged with a special tool that most tire shops will have. If the sensors cannot be registered with the tool you may have a damaged sensor that needs to be replaced. Damage to sensors can take place when tires are being mounted or if "fix-a-flat" is introduced to the tire.
This is a TSB that outlines how to reset the sensors, let me know if you have had tire work done lately and what has been done to try to reset the system
LOW TIRE INDICATOR ON - TPMS SYSTEM SENSOR WAKE UP
FORD:2003-2006 Expedition, Explorer2006 Escape Hybrid, Escape, Freestar
LINCOLN:2003-2005 Aviator2003-2006 Navigator
2006 Mariner Hybrid, Mariner, Monterey
ISSUEVehicles built with the tire pressure monitoring system (TPMS) sensors may experience a concern where the TPMS indicator is on solid and the message center (if equipped) displays "Low Tire Pressure". This may be due to the actual tire pressure being low due to time or temperature changes (10°= 1 psi). Adjusting the tire pressure to specification should turn out the warning light once the sensors have been awakened.
ACTIONIf the vehicle has been stationary for more than 30 minutes, it will be necessary to wake up the sensors with the training tool to force them to transmit the latest air pressure information to the smart junction box (SJB), or responsible module. This can be done by either driving the vehicle for 2 minutes above 20 MPH (32 Km/h) or by using the training tool. The sensors do not continuously transmit when the vehicle is not moving.
NOTE ADJUSTING THE TIRE PRESSURE IS NOT A WARRANTABLE ITEM.
If the TPMS indicator does not go off after filling the tires to the correct air pressure and pinging each sensor then refer to the appropriate Workshop Manual symptom chart.
The tool for stem mounted sensors is a circular magnet as shown below. This is the procedure to train the sensors
Sensor Training NOTE: The tire pressure sensor training procedure must be done in an area without radio frequency (RF) noise.RF noise is generated by electrical motor and appliance operation, cellular telephones, and remote transmitters.
NOTE: If the TPMS module does not recognize any one of the four tire pressure sensors during the tire training procedure, the horn will sound twice and the message center (if equipped) will display "TIRE TRAINING MODE INCOMPLETE". If this occurs, the entire procedure must be repeated from Step 1.
Let me know what your situation is and I will do my best to help you with your problem