Fault code P1520 – a step-by-step guide to diagnosing and fixing it

Error code P1520 is displayed and the check engine light is on - what does this mean?

The OBD/EOBD system detects a fault in the stop lamp switch circuit or in the electronic throttle control. This manufacturer-specific DTC P1520 mainly concerns the communication between the stop light switch and the engine control unit. The exact definition of the trouble code may vary for different manufacturers.

What causes can this fault code have?

• Defective brake light switch
• Damaged cable in the stop lamp circuit
• Loose plug connections on the brake light switch
• Faulty electronic throttle control
• Faulty fuse in the brake light circuit
• Corroded contacts in the switch area
• Malfunction in the engine control unit
• Interrupted ground connection

What symptoms occur in parallel with fault code P1520?

• Starting problems: The vehicle may not start
• Brake lights not working: The brake lights stay on permanently or do not come on
• Idling problems: Irregular engine running at idling speed
• Shifting problems: Automatic transmission cannot be moved out of park
• Cruise control failure: Cruise control system does not work
• Increased fuel consumption: Engine does not run optimally

How to fix the problem of error code P1520?

• The repair requires a systematic diagnosis of the affected circuit.
• Replace the brake light switch completely and adjust it correctly, as this is often the main cause of fault code P1520
• Check all cable connections in the stop lamp circuit and repair or replace damaged cables
• Clean the plug connections on the brake light switch, treat with contact spray and tighten firmly
• Check fuses in the brake light circuit and replace defective fuses
• Check earth connections on the switch and engine control unit and replace if necessary
• Test electronic throttle valve control with diagnostic device and replace if defective
• Check engine control unit for software errors and update software if necessary