P0504 Brake switch A/B correlation
What P0504 means
The brake pedal switch sends two separate signals to the engine control unit. When the pedal is pressed both should change together; here they do not match, so the ECU cannot be sure whether the brake is applied.
P0504 is a generic SAE J2012 code: it means the same on every make. The exact cause still depends on the car, so read the freeze-frame data before replacing parts.
At a glance
- Severity
- Low
- Typical repair cost
- 30–200 €
- Workshop time
- 0.2–1 h
- Repair difficulty
- Easy
- DIY possible
- Yes
- Check engine light
- Yes
- Affects emissions test
- No
Parts and labour at an independent workshop in Europe. Main dealers charge more; the cause decides the bill, not the code.
Symptoms
- Cruise control not working
- Brake lights not working or staying on
- Push-button start may not respond on some cars
- Gear selector hard to move out of Park on some automatics
Common causes
- Worn or failed brake light switch
- Switch out of adjustment or plastic plunger broken
- Blown brake light fuse or failed bulbs on cars that monitor that circuit
- Poor connection at the switch
Can I keep driving with P0504?
This is a low-priority fault for driving, but check that your brake lights work, because following drivers depend on them. Have the switch looked at at your next service or sooner if the brake lights are affected.
What to check first
- Ask someone to confirm the brake lights come on and go off with the pedal
- Check the brake light fuse and bulbs
- Watch both brake switch signals in live data while pressing the pedal
- Adjust or replace the brake light switch
Related codes
- P0503 Vehicle speed sensor A intermittent/erratic/high
- P0505 Idle air control system
- P0502 Vehicle speed sensor A circuit low
- P0506 Idle control system RPM lower than expected
- P0501 Vehicle speed sensor A range/performance
- P0507 Idle control system RPM higher than expected
- P0500 Vehicle speed sensor A circuit
- P0510 Closed throttle position switch
Data: TORKO fault-code catalogue, built on OBDex (CC0-1.0) and reviewed against workshop practice. Informational only — it does not replace a diagnosis by a mechanic.
Updated: