P0504 moderate

Brake Switch A/B Correlation

The P0504 code means the vehicle's onboard diagnostic system has detected: Brake Switch A/B Correlation. This is a moderate severity code.

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Keep driving?: Yes, but fix soon
DIY difficulty: easy
Estimated cost: $30 - $150 (parts and labor)

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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

Check both brake switch signals with a scan tool -- both should change state together when pedal is pressed
Inspect brake light switch at the top of the brake pedal for proper adjustment and operation
Test brake switch connector for corrosion or loose pins -- wiggle while monitoring signals
Verify brake lights turn on and off properly -- check both high-mount and rear lights
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.