Brake Switch A/B Correlation
What does P0504 mean?
The P0504 code means the vehicle's onboard diagnostic system has detected: Brake Switch A/B Correlation. This is a moderate severity code.
Common Symptoms
- Cruise control not engaging or disengaging
- Brake lights staying on or not working
- Shift interlock not releasing from park
- Push-button start not recognizing brake pedal
- Check engine light on
Probable Causes (Ranked by Likelihood)
- Faulty or misadjusted brake light switch Very High
- Brake switch plunger worn or broken High
- Corroded brake switch connector Moderate
- Wiring issue between dual brake switch circuits Moderate
- Brake pedal bushing or bracket wear changing pedal position Low
Step-by-Step Diagnostic Procedure
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Check both brake switch signals with a scan tool -- both should change state together when pedal is pressed
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Inspect brake light switch at the top of the brake pedal for proper adjustment and operation
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Test brake switch connector for corrosion or loose pins -- wiggle while monitoring signals
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Verify brake lights turn on and off properly -- check both high-mount and rear lights
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Measure voltage on both brake switch circuits -- should see 12V when brake is pressed, 0V when released
Frequently Asked Questions
Can I drive with code P0504?
Yes, you can drive, but your cruise control may not work and in some vehicles you may have difficulty shifting out of park. Ensure your brake lights are working properly for safety, as this code can sometimes affect brake light operation.
How do I adjust the brake light switch?
The brake light switch is located at the top of the brake pedal arm. Most switches are adjustable by rotating or sliding them in a bracket. The switch should click when the brake pedal is pressed about 1/4 inch and release when the pedal returns to rest.
Why does my car have two brake switch circuits?
Modern vehicles use dual brake switch circuits (A and B) for redundancy and safety. One circuit controls the brake lights, while the other communicates with the PCM for cruise control, shift interlock, and engine management. The PCM monitors both to ensure they agree.