P0504: Brake Switch "A"/"B" Correlation
Quick Answer
Not safe to drive
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.
- Severity
- Moderate · Electrical
- Estimated repair cost
- From €20 to €150+, varies by vehicle
€20 or €150, depending on your car.
- Adjust brake pedal position€20–€60
- Repair brake switch wiring and connector€45–€150
A scan shows whether P0504 is really stored on your car, which codes sit next to it and how urgent each one is. You can do this parked, with the ignition on.
A scan needs a Bluetooth OBD2 adapter (Bluetooth LE on iPhone), about €25-60; the built-in demo runs without one. See which adapter to buy
What does P0504 mean?
P0504 is a electrical-related diagnostic trouble code classified as moderate severity.
What are the symptoms of P0504?
- 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?
Failed brake light switch (internal contacts disagree)
: high
Brake pedal misadjusted, holding the switch slightly pressed
: medium
Damaged wiring or connector at the switch
: low
ECU or BCM software fault (very rare)
: low
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?
- 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
- 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
- Remove the connector and inspect for bent pins or corrosion. Reseat firmly and recheck
- If both signals stay correlated only when the connector is wiggled, repair the wiring. Otherwise, replace the brake switch as a unit
- 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 | 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 October 2026. Costs vary by region, vehicle, and shop.
P0504 on specific vehicles
BMW
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.
P0504 on Volkswagen
Volkswagen Golf
P0504 on Mercedes-Benz
Mercedes C-Class
P0504 on Audi
Related diagnostic codes
- P0571
- P0572
- P0573
- P0703
- U0140
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.