P0502: Vehicle Speed Sensor "A" Circuit Low Input
P0502 means Vehicle Speed Sensor "A" Circuit Low Input. The vehicle speed sensor (VSS) signal is used for the speedometer, transmission shifting, cruise control and more. The signal voltage is lower than the minimum the module expects. Typical causes are a short to ground, an open in the reference or signal wire, or a failed sensor.
Can I keep driving? Drivable with care. Diagnose soon: performance, economy or other components can be affected.
Common symptoms
- Speedometer erratic or not working
- Harsh or incorrect automatic shifts
- Cruise control not working
- ABS or traction control light on (if speed comes from wheel sensors)
Possible causes
- Failed vehicle speed sensor
- Damaged wiring or connector
- Damaged tone ring or drive gear
- ABS wheel speed sensor fault (on vehicles that use it for vehicle speed)
How to diagnose P0502
- Check whether the speedometer works.
- Check for ABS wheel speed codes.
- Inspect the sensor and connector.
- Check the sensor signal while the wheels turn.
What the code means, digit by digit
Generic codes have the same definition on every OBD-II vehicle (petrol cars from model year 1996 in the US, 2001 in the EU; diesels later). Manufacturers can add their own enhanced codes; those are not listed here.
Related codes
General guidance only. Fault codes describe what the engine computer saw, not which part failed: confirm with testing before replacing parts, and follow the vehicle's service information.