Transfer Case Speed Sensor
WARNING: This page does not describe the selected car, but rather 31 other vehicles, including the 2003 Mercury Sable, 2003 Mercury Mountaineer, 2003 Mercury Marauder, 2003 Mercury Grand Marquis, and 2003 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.
See TEST DP: VEHICLE SPEED SENSOR/TRANSFER CASE SPEED SENSOR under SYSTEM TESTS in SELF-DIAGNOSTICS - CNG, FLEX-FUEL & GASOLINE article for Transfer Case Speed Sensor (TCSS) testing procedures and specifications.