Vehicle Speed Sensor A Circuit Low Input
What does P0502 mean?
The P0502 code means the vehicle's onboard diagnostic system has detected: Vehicle Speed Sensor A Circuit Low Input. This is a moderate severity code.
Common Symptoms
- Speedometer reads zero while driving
- Transmission stuck in one gear or shifts harshly
- Cruise control inoperative
- ABS warning light on
- Check engine light illuminated
Probable Causes (Ranked by Likelihood)
- Open or shorted wiring in VSS circuit Very High
- Faulty vehicle speed sensor High
- Corroded or disconnected VSS connector High
- Damaged VSS signal wire (shorted to ground) Moderate
- PCM input circuit failure Low
Step-by-Step Diagnostic Procedure
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Check VSS connector for damage, corrosion, or loose pins -- clean and reseat
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Measure voltage at VSS signal wire with vehicle moving -- should see pulsing 0-5V signal
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Test VSS signal wire for shorts to ground -- disconnect sensor and check resistance to ground (should be infinite)
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Inspect wiring harness routing for chafing against transmission housing or exhaust components
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Verify VSS power supply voltage -- should be 5V or 12V reference depending on sensor type
Frequently Asked Questions
Can I drive with code P0502?
You can drive short distances carefully, but the transmission may not shift properly and the speedometer will be inaccurate. The vehicle may default to a single gear (limp mode) for transmission protection. Avoid highway driving.
What causes a low input signal from the speed sensor?
A low input typically means the signal wire is shorted to ground, the sensor has failed internally, or the wiring connection is broken. The PCM sees a voltage lower than expected on the VSS signal circuit.
Can I replace the speed sensor myself?
Yes, on many vehicles the VSS is accessible from underneath the car on the transmission housing. It usually requires a 10mm socket and the sensor pulls straight out. However, some vehicles require removal of other components for access.