P0502: OBD-II code P0502 flags a low or missing signal from the vehicle speed sensor circuit — the PCM sees no speed input at all.

Symptoms

• Check engine light is on; the speedometer reads zero even while the vehicle is moving.
• Cruise control is disabled and the transmission may hold gears too long or shift harshly.
• The ABS warning lamp may also illuminate on vehicles that share the same speed input.

Common causes

• A failed VSS with an open internal element producing no signal.
• A broken signal wire or disconnected sensor — common under the vehicle after off-road driving.
• On CAN-based vehicles, an ABS module fault or bus issue preventing speed messages from reaching the PCM.

What to do?

Test the VSS with a DMM (AC voltage while spinning the driveshaft). On newer CAN-based cars, scan every module for U-codes and ABS wheel-speed codes first. Inspect wiring at the sensor. Replace the sensor or repair wiring.