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DTC/P0504

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

Quick Answer
Moderate SeverityNot safe to driveElectrical

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.

Estimated repair costFrom €20 to €150+, varies by vehicle

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What does P0504 mean?

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.

What are the symptoms of P0504?

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

What causes P0504?

Here are the most common causes of P0504:

high: Failed brake light switch (internal contacts disagree)
medium: Brake pedal misadjusted, holding the switch slightly pressed
low: Damaged wiring or connector at the switch
low: ECU or BCM software fault (very rare)

Is it safe to drive with P0504?

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.

How do you diagnose P0504?

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

How much does P0504 cost to fix?

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

Repair Parts Cost Labor Cost Total Estimate DIY Difficulty
Adjust brake pedal position €0–€0 €20–€60 €20–€60 Easy
Replace brake light switch €15–€40 €20–€60 €35–€100 Easy
Repair brake switch wiring and connector €5–€30 €40–€120 €45–€150 Moderate

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

P0504 on specific vehicles

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

BMW 3 Series

2005-2024

F30 3 Series with push-button start commonly fail to crank when the brake light switch loses correlation. The switch itself is inexpensive, but the symptom (no-start) often sends owners to a tow truck before the diagnosis is made.

Volkswagen

P0504 on Volkswagen

Volkswagen Golf

See Volkswagen-specific causes & cost
Mercedes-Benz

P0504 on Mercedes-Benz

Mercedes C-Class

See Mercedes-Benz-specific causes & cost
Audi

P0504 on Audi

Audi A4

See Audi-specific causes & cost

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

P0571P0572P0573P0703U0140

Common questions about P0504

Why won't my car start after the brake light switch failed?

Push-button start vehicles require the brake pedal to be pressed before the engine will crank. The ECU checks the brake switch signal to confirm the pedal is depressed. If P0504 is set, the ECU cannot trust that signal and refuses to start as a safety measure.

Can I just replace the brake light switch myself?

On most cars yes. The switch is a small plastic part that twists or clips into a bracket above the brake pedal. The job typically takes 10 to 15 minutes and requires no special tools. Just disconnect the battery before unplugging the switch to avoid airbag warnings.

Why are my brake lights stuck on all the time?

A stuck-on circuit inside a failing brake switch is one of the classic P0504 symptoms. The pedal-released contact has welded closed, so the brake lights stay on even with the pedal up. Replace the switch to fix.

Will a recalled brake switch cause P0504?

Possibly. Brake light switches have been the subject of manufacturer recall campaigns, and a switch whose two internal contacts disagree can set P0504 along with erratic brake light behaviour. Check your vehicle's recall status by VIN with the manufacturer or your national authority before paying for parts.

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