Fault code P0703 – a step-by-step guide to diagnosing and fixing it

Error code P0703 is displayed and the check engine light is on - what does this mean?

The OBD system detects a fault in the brake light switch circuit "B", which is sending an incorrect or missing signal to the engine control unit. This DTC P0703 affects the communication between the brake pedal and various vehicle systems such as the shift lock in automatic transmissions. For some manufacturers, this error code can also be referred to as "Brake Switch B Circuit Malfunction".

What causes can this fault code have?

• Defective brake light switch
• Damaged wiring to the brake light switch
• Corroded or loose plug connections
• Incorrectly adjusted brake light switch
• Defective fuse in the brake light circuit
• Faulty earth connection
• Defective engine control unit (rare)

What symptoms occur in parallel with fault code P0703?

• Brake lights do not work: The brake lights do not light up when the brake pedal is pressed
• Shift lock activated: In automatic transmissions, the selector lever cannot be moved out of the park position
• Cruise control not working: The cruise control cannot be activated or deactivated
• Start-stop system deactivated: The automatic engine shutdown does not work
• ESP warning light: Electronic stability programs may be impaired
• Irregular engine behavior: Engine may be running in emergency mode

How can the problem with error code P0703 be resolved?

• Various repair approaches are available to rectify trouble code P0703:
• Replace the brake light switch: Remove the defective switch on the brake pedal and replace it with a new original part, then adjust it correctly
• Repair the wiring: Locate damaged cables to the brake light switch, repair severed wires or replace complete wiring harnesses
• Clean plug connections: Clean corroded contacts on the brake light switch with contact spray and tighten connections firmly
• Check and replace fuses: Check all fuses in the brake light circuit and replace defective fuses with new ones
• Readjust the switch: Adjust the existing brake light switch mechanically correctly so that it switches properly on brake pedal contact
• Check ground connections: Locate poor ground connections, clean contact points and tighten screw connections