P0502 Vehicle Speed Sensor A Circuit Low Input

P0502 indicates a low input from vehicle speed sensor A. The module sees a speed signal lower than expected or missing during conditions where speed should be present.

Code details

Generic OBD-II · P0

System: Speed / Idle
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not diagnose shift or cruise problems before verifying the speed signal; many systems depend on the same input.

Symptoms of P0502

Unstable, high, or low idle, or stalling.
Erratic speedometer, cruise-control, or shift behaviour.
Check-engine light with a hesitation or surge.

Common causes of P0502

A dirty throttle body, idle-control valve, or throttle-position sensor.
A failing vehicle-speed or wheel-speed sensor, or its wiring.
Vacuum leaks or a related sensor forcing the idle/speed strategy.

Useful checks before replacing parts

Compare scan-tool vehicle speed with actual road speed and other module speed signals.
Inspect sensor power, ground, signal, connector condition, harness routing, and reluctor or tone-wheel damage.
Check for transmission, ABS, or cluster codes that share vehicle-speed information.

Related OBD-II codes

These nearby speed / idle entries can help compare wording and pin down the fault area.