Fault code P0503 – a step-by-step guide to diagnosing and fixing it

Error code P0503 is displayed and the check engine light is on - what does this mean?

DTC P0503 is triggered by the OBD/EOBD system when an intermittent or irregular speed sensor is detected. This fault code indicates problems with the vehicle speed sensor (VSS), which sends implausible or fluctuating signals to the engine control unit. With various manufacturers, this code can also indicate problems with the ABS system or the speedometer display.

What causes can this error code have?

• Defective vehicle speed sensor (VSS)
• Damaged or corroded wiring to the speed sensor
• Loose or dirty plug connections
• Defective ABS control unit
• Damaged sensor wheel on the differential or gearbox
• Faulty ground connection of the sensor
• Defective speedometer cable or drive components
• Malfunctions in the vehicle electrical system or electromagnetic interference

What symptoms occur in parallel with fault code P0503?

• Speedometer problems: The speedometer displays incorrect or fluctuating speed values or fails completely
• Shifting problems: Automatic transmission shifts erratically or stalls in a gear
• Cruise control failure: The cruise control no longer works or switches off automatically
• ABS problems: Anti-lock braking system malfunctions or the ABS warning light comes on
• Engine performance: Reduced engine power or rough engine running at certain speeds

How to fix the problem of error code P0503?

• Repairing the P0503 trouble code requires a systematic diagnosis of the speed sensor components.
• Replace the speed sensor: Remove the faulty VSS from the transmission or differential and replace it with a new OEM sensor, making sure to follow the correct torque specification
• Repair wiring: Locate damaged wiring to speed sensor, repair severed or corroded areas and seal with electrical tape or heat shrink tubing
• Clean plug connections: Clean all plug contacts on the sensor and control unit with contact cleaner, check for tight fit and use new plugs if necessary
• Check the sensor wheel: Check the pulse wheel on the differential for damage, dirt or loose fit and clean or replace if necessary
• Replace the ground connection: Identify defective ground points on the speed sensor, remove corrosion and lay new ground wires
• Carry out a control unit reset: After all repairs, delete the fault memory, complete a test drive and check the system for correct function