Possible causes

– Faulty Vehicle Speed Sensor
– Vehicle speed sensor harness or connectors is open or shorted
– Vehicle speed sensor circuit poor electrical connection
– Faulty Powertrain Control Module (PCM)

When is the code detected?

When the Powertrain Control Module (PCM) detects an unrealistically large drop in the vehicle speed, then DTC P0503 sets.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Speedometer may not work properly
– Transmission shifting problem

P0503 GMC Description

The Vehicle Speed Sensor (VSS) assembly provides vehicle speed information to the Powertrain Control Module (PCM). The VSS assembly is a Permanent Magnet (PM) generator. The PM generator produces a pulsing AC voltage as rotor teeth on the transmission output shaft pass through the sensor’s magnetic field. The AC voltage level and the number of pulses increase as the speed of the vehicle increases. The PCM converts the pulsing voltage to vehicle speed. The PCM uses the vehicle speed signal to determine shift timing and torque converter clutch (TCC) scheduling.