What Does P0504 Mean?
The PCM has detected a correlation error between brake switch 'A' and brake switch 'B' — one switch indicates the brake pedal is pressed while the other indicates it is released. Modern vehicles use two brake switches (or a dual-function switch) for redundancy: one for the brake lights and one as a PCM input for cruise control disengagement and shift interlock. The driver may notice cruise control not working, the brake lights staying on or not illuminating, or shift interlock issues.
Common Causes
35%
Misadjusted brake light switch causing one circuit to activate before or after the other
30%
Failed brake light switch with one internal circuit stuck open or closed
20%
Broken brake pedal bumper/stopper pad causing switch misalignment
15%
Wiring issue in brake switch circuit causing voltage drop or open in one leg
Diagnostic Steps
1
Monitor both brake switch PIDs simultaneously on the scan tool while slowly pressing and releasing the brake pedal — both should transition together within a very small pedal travel window.
2
Visually inspect the brake light switch at the top of the brake pedal arm. Check that the switch plunger is properly contacting the brake pedal bracket and that the retaining clip is secure.
3
Check for a missing or broken brake pedal bumper/stopper pad — this small rubber or plastic piece prevents the switch from over-extending and is a very common failure item.
4
Measure voltage on both brake switch output circuits with the pedal pressed and released — each circuit should show proper switching between 0V and battery voltage.
5
If the switch is adjustable, follow the manufacturer's adjustment procedure to ensure both circuits engage within the specified pedal travel.
Estimated Repair Cost
$15 - $120
Parts + labor, varies by vehicle and location
The PCM has detected a correlation error between brake switch 'A' and brake switch 'B' — one switch indicates the brake pedal is pressed while the other indicates it is released. Modern vehicles use two brake switches (or a dual-function switch) for redundancy: one for the brake lights and one as a ...
The most common cause of P0504 (Brake Switch "A"/"B" Correlation) is: Misadjusted brake light switch causing one circuit to activate before or after the other
Typical repair costs for P0504 range from $15 to $120, depending on the vehicle, location, and whether you do it yourself or go to a shop.
This code can indicate that brake lights are not functioning correctly, which is a significant safety concern for following vehicles. Cruise control may fail to disengage when braking. The vehicle can be driven, but brake light operation should be verified immediately — have someone observe the brake lights while you press the pedal.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0504 to identify the root cause.
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Quick Info
Category
Powertrain
System
Brake System / Cruise Control
Difficulty
Type
Generic (SAE)
Recommended Tools
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