P0504 — Brake Switch A/B Correlation
Moderate severityWhat it means
P0504 is stored when the engine control unit sees a fault involving the brake switch signal. The brake switch tells the control unit when the brake pedal is pressed, which it needs for cruise control, shift interlock and idle control. The control unit has flagged this circuit or signal as outside its normal operating range.
Common symptoms
- Cruise control will not engage
- Shifter locked in Park in some cases
- Brake lights staying on or not working
Likely causes
- A wiring, connector or ground problem in the circuit
- A failed sensor or actuator in the circuit
- A misadjusted or faulty brake light switch
- A wiring or connector fault
What to check first
- Inspect the connector and wiring for damage, corrosion or loose pins
- Watch the brake switch state toggle in live data when you press the pedal
- Check the switch adjustment and connector
Can you keep driving?
Drivable, but cruise control and the shift interlock may not work; fix it soon for safety.
Frequently asked questions
What is the most common cause of P0504?
The most likely cause is: A wiring, connector or ground problem in the circuit. Several other faults can set P0504 as well, so confirm the real cause with live data and the checks above before replacing any parts.
Can I just clear P0504 and keep driving?
Clearing P0504 with an OBD2 scanner turns the warning light off, but if the underlying fault has not been repaired the code will usually come back within a few drive cycles. Fix the cause first, then clear the code to confirm the repair worked.
How serious is P0504?
P0504 is a moderate-severity code. The car is often still drivable, but have it diagnosed soon — the underlying problem can get worse and it may fail an emissions test.
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