P0504: Brake switch A/B correlation
The brake pedal switch sends two signals that should agree and do not. Usually the switch itself is out of adjustment or faulty.
| Code | P0504 |
|---|---|
| System | Engine and transmission |
| Group | P05xx, Speed, idle and auxiliary inputs |
| Type | Generic: it means the same on every make |
| Can you keep driving? | With care |
What P0504 means
The brake pedal switch has two contacts: one lights the brake lights and the other tells the ECU. When you press or release the pedal both should change together. If they do not match, the ECU cannot be sure whether you are braking.
Most common causes
- Brake pedal switch out of adjustment
- Faulty brake switch
- Broken or missing rubber stop on the pedal
- Blown brake light fuse
- Damaged switch connector or wiring
Symptoms
- Brake lights not coming on or staying on
- Cruise control not engaging or cutting out
- Difficulty moving the lever out of P on automatics
- No response when starting cars that need the brake pressed
- Engine warning light on
Can you drive with P0504?
With care The brakes work as normal, but if the brake lights fail, drivers behind cannot see you slowing down. Check them and get it fixed soon.
How to check it
Ask someone to watch the brake lights while you press the pedal. On the scanner, watch both brake switch states live: they should change together as you press and release.
How to read and clear P0504 with Final OBD
Plug an OBD2 scanner in under the steering wheel, switch the ignition on and open Final OBD. Under Faults you’ll see P0504 with this same explanation and whether it’s confirmed, pending or permanent.
With Free you read this code with its short description for nothing, and clear it if the engine stores it over standard OBD. With Pro you get the full explanation, how the car was running when it was logged (revs, temperature, load) and clearing in every control unit.
Clearing the code only turns the light off: if the cause is still there, it comes back. Clear it once it’s fixed, and not right before the ITV. It’s all in how to read and clear fault codes and in the ITV and OBD.
Similar codes
- P0500 Vehicle speed sensor malfunction
- P0501 Vehicle speed sensor range/performance
- P0502 Vehicle speed sensor circuit low input
- P0503 Vehicle speed sensor intermittent, erratic or high
- P0505 Idle control system malfunction
- P0506 Idle speed lower than expected
- P0507 Idle speed higher than expected
- P0513 Incorrect immobiliser key
- P0520 Oil pressure sensor/switch circuit malfunction
- P0521 Oil pressure sensor/switch range/performance
- P0522 Oil pressure sensor/switch low voltage
- P0523 Oil pressure sensor/switch high voltage
P0504 is a generic code, so it means the same on any make, for example on Citroën, Hyundai, Mini, Renault, Seat, Volvo. See what Final OBD does on your car in makes and models.
What does code P0504 mean?
The brake pedal switch has two contacts: one lights the brake lights and the other tells the ECU. When you press or release the pedal both should change together. If they do not match, the ECU cannot be sure whether you are braking.
Can you drive with P0504?
The brakes work as normal, but if the brake lights fail, drivers behind cannot see you slowing down. Check them and get it fixed soon.
How do you clear P0504?
With an OBD2 scanner and an app like Final OBD, which clears it for free if it’s stored by the engine control unit. But clearing it only turns the light off: if the cause isn’t fixed, P0504 comes back.
Read it on your car
Final OBD reads the faults of every control unit for free, including each make’s own codes, and with Pro explains each one in plain English. Free clears engine faults at no cost.
Coming soon
Mid to late October 2026
If you want to try it in your car before anyone else, send us your email and we will add you as a tester.
info@finalobd.com