P0504
PowertrainBrake Switch A/B Correlation
Your brake pedal switch sends two separate signals to the car's modules, and they're meant to agree with each other every time you touch the brake. P0504 logs when those two signals disagree. For you that usually shows up as dodgy brake lights, cruise control that won't stay on, or an automatic that fights you when you try to come out of Park. It's almost always electrical, and most of the time the brake switch itself or its connector is the culprit rather than anything expensive.
ⓘ Information only. This page provides general educational information about fault code P0504. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0504 mean?
P0504 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Brake Switch A/B Correlation.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Brake lights stuck on permanently, or not coming on at all when you press the pedal
- • Cruise control won't engage, or drops out the moment you touch the brake
- • Automatic box refuses to shift out of Park, since the shift interlock relies on the brake signal
- • Engine warning light on the dash, sometimes alongside ABS or stability control lamps
- • Start-stop systems and adaptive cruise going offline on newer cars
- • Occasional stall or stumble when braking from speed if the signal glitches badly
Possible causes
Causes commonly associated with P0504, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Faulty dual-circuit brake pedal switch. This is the usual suspect, and the switch is a cheap, common-failure part on a lot of VAG, BMW and Ford models
- 2. Corroded or pushed-back pins in the brake switch connector, water and road salt find their way in over the years
- 3. Chafed or broken wiring in the brake switch harness behind the pedal box
- 4. Blown brake light fuse from a short or moisture getting in, which kills one circuit and leaves them mismatched
- 5. A failed brake light bulb on cars where the BCM watches bulb current, the imbalance can trip the correlation check
- 6. Body control module out of calibration after a battery change or repair, so it reads the switch states wrongly
- 7. ECU or network communication fault misreading the switch data, rare but it does happen
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read the codes and freeze frame data, and check brake light operation first. Have someone press the pedal while you watch the lights come on and go off cleanly. A lot of P0504s are nothing more than a duff bulb or a stuck switch
- 2. Pull the brake light fuse and test it for continuity, then check every brake bulb including the high-level third light for a blown filament or water in the lens
- 3. Get under the dash to the brake switch connector and inspect the pins for green corrosion, spread terminals or pushed-back pins. Wiggle-test the harness while watching live data
- 4. Back-probe the switch terminals and measure the voltage with the pedal up and down, comparing both circuits against the service manual figures. The two signals should flip together
- 5. If the bulbs, fuse and wiring all check out and the two outputs still don't agree, the switch is the likely fault. Fit a new one and clear the code, then road test to confirm it stays gone
Common questions about P0504
Can I sort this out myself without a garage? +
Often yes, if it's the simple stuff. Check the brake bulbs and the brake light fuse first, because a blown bulb or fuse can throw this code and costs pennies to put right. The brake switch is a bolt-in part on most cars (£15 to £60) but it sits up behind the pedal in a cramped footwell, and you'll want a multimeter to confirm both circuits are misbehaving before you buy one. If it's corroded wiring or a BCM calibration issue, that's garage territory.
If I just clear the code, will it stay gone? +
Only if the fault was a one-off glitch, which is uncommon with P0504. If the switch is failing or a connector is corroded, the code comes straight back, sometimes within a few miles, sometimes the next time you brake hard. Clearing it without fixing the cause also leaves your brake lights playing up, so you're driving around with a real safety problem regardless of what the dash says.
What's the risk if I just leave it? +
This isn't one to ignore. The same circuit feeds your brake lights, so the driver behind may get no warning when you slow down, and that's a rear-end shunt waiting to happen. You'll likely lose cruise control, your auto box may refuse to come out of Park, and on some cars the signal confusion can cause a stall or rough running when you brake. None of that gets better on its own.
How quickly do I need to deal with it? +
Treat it as a priority job, not something to put off for weeks. Brake lights are an MOT item and a safety item, so if they're stuck on or dead you want it fixed within days, not next month. The diagnosis is usually quick and the parts are cheap, so there's little reason to leave it hanging. If the brake lights are still working normally and it's only cruise control affected, you've got a bit more breathing room, but still get it looked at soon.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →