P0502 - Vehicle Speed Sensor (VSS) 'A' Circuit Low
System
Powertrain (Engine & Gearbox)
Technical Description
The control module has detected a fault in the Vehicle Speed Sensor (VSS) 'A' Circuit Low (P0502). The signal is below the lower threshold, typically from a short to ground or an internally shorted sensor.
Multimeter Test Points & Nominal Values
| Test Point / Component | Measurement | Nominal Target Range | Test Condition |
|---|---|---|---|
| Vehicle Speed Sensor (VSS) 'A' Circuit Low – Supply Voltage | Voltage (V) | 5.0 V or 12.0 V (type dependent) | Ignition ON at connector |
| Vehicle Speed Sensor (VSS) 'A' Circuit Low – Signal Low / High Level | Voltage (V) | Low < 0.5 V / High > 4.5 V (square-wave toggle) | Engine barred over slowly |
| Signal Ground Voltage Drop | Voltage (V) | < 0.05 V | Under load to ground |
Ignition ON at connector
Engine barred over slowly
Under load to ground
Oscilloscope Waveform Analysis
1 ms/div, 2 V/divPossible Causes
Short to ground on the signal wire or an internal short circuit within Vehicle Speed Sensor (VSS) 'A' Circuit Low
Channel A:Verify dedicated wiring pins and signal pair for circuit A against the schematic.
Chafed or pinched wiring harness along the signal path to Vehicle Speed Sensor (VSS) 'A' Circuit Low
Poor ground connection or excessive voltage drop (> 0.1 V) at the ground point for Vehicle Speed Sensor (VSS) 'A' Circuit Low
Defective internal output driver or reference circuit in the control module for Vehicle Speed Sensor (VSS) 'A' Circuit Low
Symptoms
Recommended Solution
Compare signal voltage and resistance between circuit A and companion circuit B at module connector.
Measure signal wire resistance to chassis ground with connector unplugged (must exceed 10 kΩ; low reading confirms short to ground).
Verify loaded supply voltage and harness terminal pins at Vehicle Speed Sensor (VSS) 'A' Circuit Low.
Check circuit continuity and ground-side voltage drop (< 0.05 V) for Vehicle Speed Sensor (VSS) 'A' Circuit Low.
Inspect the reluctor target and sensor mounting for radial runout, wobble, or incorrect air gap.
Mechanic's Tip
If the Hall signal is stuck at 5V or 0V while rotating, unplug the sensor and probe the signal wire to ECM: seeing 5V pull-up confirms harness and ECM integrity, pinpointing a dead Hall IC.