P0504: Brake Switch A/B Correlation

What it means

Modern vehicles use two redundant brake switch signals (often one normally-open and one normally-closed). When the two signals disagree, this code is set; common causes include a misadjusted switch, internal switch failure, or wiring fault.

At a glance

• Usually lights the check-engine light

How it works

The brake pedal switch provides redundant A and B signals so the control module can verify when the pedal is pressed and released. P0504 sets when those signals do not match the expected relationship, which can affect brake lights, cruise control, or push-button starting.

Common causes

Most likely first.

1. Brake switch misadjusted
2. Brake switch internal fault

Symptoms

• Check-engine light on
• Brake lights may stay on or fail to illuminate
• Cruise control may not work
• Push-button starting may be affected

How to diagnose it

1. Use a scan tool to check brake-switch A and B live data while slowly pressing and releasing the pedal, noting whether either signal changes late or fails to change.
2. Check that the brake lights turn on with pedal pressure and go out when the pedal is released.
3. Inspect the brake switch at the pedal for incorrect adjustment, damage, a loose connector, or corroded pins.
4. Check the switch wiring for chafing, broken conductors, or shorts, especially near the pedal and connector.
5. Test switch operation and circuit continuity against the manufacturer's specification before replacing the switch.

Common fixes

• Adjust the brake switch so it changes state at the correct pedal position.
• Clean and secure the brake-switch connector.
• Repair damaged or shorted wiring in the A or B signal circuit.
• Replace a brake switch that fails its electrical test.

Can I keep driving?

Check the brake lights before driving; if they stay on or fail to illuminate, avoid driving because other drivers may not see your braking. Cruise control or push-button starting may also be unreliable.