P0504
Brake Switch "A"/"B" Correlation
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The P0504 code means the powertrain control module (PCM) has detected a disagreement between the two brake switch circuits. Modern vehicles use a brake switch with two separate circuits — one for the brake lights and one that tells the PCM the brakes are applied. When these two signals don't agree (one says the brake is pressed while the other says it isn't), the PCM sets this code.
This is a safety-critical code that should be addressed promptly. You may notice your brake lights stuck on, draining your battery and confusing other drivers, or they may not come on at all when you press the brake pedal — a serious hazard. Cruise control may not function because it relies on the brake switch signal to disengage. Some vehicles won't shift out of park without a proper brake switch signal, leaving you stranded.
The good news is that the fix is usually simple and inexpensive. The brake light switch is located at the top of the brake pedal arm under the dashboard. It either needs to be readjusted or replaced entirely — both are straightforward DIY jobs. Before replacing the switch, check that all brake light bulbs are working and the fuse is intact, as these can sometimes cause the correlation fault. Given the safety implications of faulty brake lights, don't delay this repair.
Severity
Symptoms
- •Brake lights stay on when the pedal is released or don't illuminate when pressed
- •Cruise control refuses to engage or won't cancel when braking
- •Vehicle won't shift out of park in some models
- •Engine may stall when coming to a stop at highway speeds
- •Shift interlock system does not function properly
Likely Causes
The brake light switch has two internal circuits (A and B) that should agree. When the switch fails or becomes misadjusted, these circuits send contradictory signals to the PCM.
The switch mounting position has shifted so it doesn't fully engage or disengage with the brake pedal arm, causing the two circuits to read inconsistently.
Damaged, corroded, or loose wiring between the brake switch and the PCM causes one circuit to read differently than the other.
A blown fuse or burned-out brake light bulb can create a feedback voltage difference that the PCM interprets as a correlation fault between circuits A and B.
The module receiving the brake switch signals has an internal fault causing it to misinterpret valid signals as a correlation error.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the brake light switch
- Adjust the brake light switch to the correct position on the pedal bracket
- Repair or replace damaged wiring and connectors in the brake switch circuit
- Replace blown brake light fuse or burned-out brake light bulbs