P0504 Brake Switch A/B Correlation

P0504 points to a brake switch A/B correlation problem. The controller sees brake switch signals that disagree or change in an impossible sequence.

Code details

Generic OBD-II · P0

System: Speed / Idle
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not adjust or replace the brake switch without verifying both circuits and stop lamp operation.

Symptoms of P0504

Unstable, high, or low idle, or stalling.
Erratic speedometer, cruise-control, or shift behaviour.
Check-engine light with a hesitation or surge.

Common causes of P0504

A dirty throttle body, idle-control valve, or throttle-position sensor.
A failing vehicle-speed or wheel-speed sensor, or its wiring.
Vacuum leaks or a related sensor forcing the idle/speed strategy.

Useful checks before replacing parts

Compare brake switch A and B states, pedal position, cruise cancel status, shift interlock data, freeze-frame speed, and related brake or cruise codes.
Inspect brake switch adjustment, connector terminals, pedal return, blown fuses, wiring damage, stop lamp operation, and moisture intrusion.
Verify both switch circuits change cleanly and in the expected order as the pedal is pressed and released.

Related OBD-II codes

These nearby speed / idle entries can help compare wording and pin down the fault area.