P0703
Brake Switch B Circuit.
- Generic
- Powertrain
- Generic code
- Medium severity
How this code is built
- P
- SystemPowertrain
- 0
- Defined bySAE, the same on every car
- 7
- AreaTransmission
- 03
- FaultThe specific fault in that area
What it means
P0703 means the engine control unit has detected a fault in the Brake Switch B circuit, the second brake pedal switch or sensor input used alongside the main brake switch. The control unit compares the signal with brake switch A, vehicle speed and other inputs, and sets the code when the signal is missing, stuck, out of range or does not agree with what is expected.
Symptoms
- Brake lights not working, or staying on all the time
- Cruise control not working or cancelling unexpectedly
- Engine warning light on
- Car will not shift out of park, or the shift interlock feels wrong (automatics)
- Difficulty starting on models that need the brake pressed to start
- Idle or throttle response changes when braking on some models
Possible causes
- Brake pedal switch faulty or out of adjustmentlikely
- Switch plunger or pedal stop worn, missing or incorrectly positioned so the switch does not change statelikely
- Wiring damage, corrosion or a poor connection at the switch connectorlikely
- Blown brake light fuse or a fault in the brake light circuitpossible
- Fault in a body control module or other control unit that shares the brake switch signalpossible
- Faulty engine control unit input circuitrare
How to diagnose
- Read the stored codes and freeze frame data, and note any other brake, cruise control or transmission codes
- Check that the brake lights work and switch off when the pedal is released
- Inspect the brake switch, its connector and wiring for damage, water ingress, corrosion or poor pin tension
- Check the switch position and the pedal stop or plunger for wear or misadjustment, following the manufacturer's procedure
- Check the relevant fuses
- View live data and confirm both brake switch signals change cleanly when the pedal is pressed and released
- Test the switch's resistance or continuity and check supply, ground and signal wiring for opens, shorts and voltage drop, as the circuit layout depends on the model
- Check related control units and their wiring only if the switch and wiring test good, as the signal routing depends on the vehicle
How to fix
- Replace the brake pedal switch and adjust it to specification
- Repair or replace worn pedal stops, plungers or mounting parts
- Repair damaged wiring or connectors, and clean or replace corroded terminals
- Replace blown fuses after finding why they failed
- Repair or replace a faulty control unit only after the circuit has been proven good, and carry out any programming the manufacturer requires
Can I keep driving?
Medium severity
The car can usually be driven short-term, but faulty brake lights are unsafe and illegal, so check them first. Get it repaired soon, as the fault may stop cruise control, starting or gear selection.
Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.