P0502 - Vehicle Speed Sensor 'A' Circuit Low Input
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0502 is set when the Engine Control Module (ECM/PCM) detects a voltage signal from the Vehicle Speed Sensor (VSS) 'A' that is lower than the specified threshold. The VSS is typically an inductive or Hall-effect sensor located on the transmission output shaft or wheel hub. The ECM compares the VSS signal with other parameters such as engine RPM and gear ratio. If the vehicle is determined to be in motion based on RPM but the VSS signal remains near zero for a calibrated period, the DTC is stored. This fault affects speedometer operation, transmission shift logic, and stability control systems via CAN-bus data sharing.
Possible Causes
Defective Vehicle Speed Sensor (VSS) (internal short or open resistance)
Wiring issues (chafed insulation, short to ground, or open signal circuit)
Excessive buildup of metal debris on the sensor's magnetic tip
Faulty instrument cluster or internal PCM/TCM failure
Symptoms
Recommended Solution
Workflow
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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