P0504: Brake Switch "A"/"B" Correlation

P0504 means the two internal circuits of the brake light switch do not agree. Modern brake switches send two signals to the ECU for safety: one that closes when the pedal is pressed and one that opens. If the ECU sees both circuits in the same state at the same time, it sets P0504. The fix is almost always a new brake switch, sometimes combined with a pedal position adjustment.

P0504 is a electrical-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0504 and illuminates the check engine light.

If your vehicle has triggered P0504, you may notice one or more of these symptoms:

• Cruise control will not engage or drops out as soon as it is set
• Brake lights stay on continuously or do not come on at all
• Push-button start refuses to crank the engine
• Automatic transmission will not shift out of Park
• Stability control or hill-hold features disabled

Here are the most common causes of P0504, ranked by how likely they are to be the culprit:

No - address this immediately. Most modern brake pedal switches have two redundant circuits. When they disagree, the ECU cannot trust the brake pedal input. Cruise control will not work, brake lights may stay on or fail, and on push-button start vehicles the engine may refuse to start. Repair before extended driving.

Follow these steps to pinpoint the root cause of P0504:

1. Use a scan tool to view live brake switch data. Most ECUs show two signals (BLS and BTS or similar). Press the pedal and verify both change state cleanly together
2. Inspect the pedal at rest. The switch plunger should be fully extended and resting against the pedal stop with no preload. A loose stop pad lets the pedal sit too low and pre-presses the switch
3. Remove the connector and inspect for bent pins or corrosion. Reseat firmly and recheck
4. If both signals stay correlated only when the connector is wiggled, repair the wiring. Otherwise, replace the brake switch as a unit
5. Clear codes, perform any required relearn for cruise control, and confirm the brake lights work both at rest and when the pedal is pressed

Repair costs for P0504 vary depending on the root cause and your vehicle. Here are typical estimates:

P0504 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

These codes are often seen alongside P0504 or indicate related issues: