P0504
Brake Switch A / B CorrelationP0504 is a generic OBD-II powertrain diagnostic trouble code: Brake Switch A / B Correlation. It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0504 means
P0504 is a correlation fault in the brake pedal switch circuit. Almost all modern vehicles use a brake switch with two independent sets of contacts, referred to as circuit "A" and circuit "B". They are deliberately wired to behave in opposite ways: one closes when the pedal is pressed while the other opens, so the ECU always receives two complementary signals. That redundancy lets the module tell a genuine brake application apart from a stuck switch, a shorted wire or a tampered circuit, which matters because the same signal feeds brake lights, cruise control, the transmission shift interlock, stability control, hill-hold and engine stop-start.
The ECU continuously compares the two inputs. If both report pressed at once, both report released at once, or one changes state noticeably later than the other, the plausibility check fails and P0504 is stored. Because the fault is about the two signals disagreeing rather than one of them being missing, the code can appear even when the brake lights still work normally.
The most common physical causes are a worn or misadjusted brake pedal switch and, on many vehicles, a crumbled plastic stopper pad on the pedal bracket that lets the switch plunger sit in the wrong position. Wiring and connector faults are the next most likely. Because the affected systems include brake lights, cruise control and the shift interlock, this is treated as a safety-relevant fault and should be addressed promptly.
Common causes
Most-frequently reported root causes when P0504 is logged.
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1
A worn, sticking or internally failed brake pedal switch, by far the most common cause.
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2
A brake pedal switch that has slipped out of adjustment, or a crumbled plastic stopper pad on the pedal bracket that changes where the plunger rests.
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3
Corroded, loose or backed-out terminals in the brake switch connector.
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4
Chafed, broken or shorted wiring between the brake switch and the engine or body control module.
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5
A blown fuse or poor ground affecting one of the two switch circuits but not the other.
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6
Failed brake light bulbs or corroded bulb holders on vehicles where the module monitors lamp current on one circuit.
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7
Water intrusion into the switch or connector from a leaking windscreen or bulkhead seal.
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8
Rarely, a failure inside the control module that reads the two brake switch inputs.
Symptoms drivers notice
How to diagnose P0504
A typical diagnostic flow when this code is present.
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1
Read all stored codes and freeze-frame data, and note any brake, stability control or transmission codes stored alongside P0504.
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2
Have someone press the brake pedal while you watch the brake lights and confirm they come on and go off cleanly, then check the fuse and all brake light bulbs and holders.
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3
Monitor the Brake Switch A and Brake Switch B parameters in live data while slowly pressing and releasing the pedal, and look for one signal changing state late, early or not at all.
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4
Inspect the brake pedal bracket for a missing or crumbled plastic stopper pad, and check that the switch plunger is correctly positioned and adjusted.
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5
Unplug the switch connector and inspect both sides for corrosion, spread or pushed-back pins, and moisture.
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6
Check the switch's supply, ground and both signal circuits for voltage and continuity at the module connector against the manufacturer's specifications, wiggling the harness to expose intermittent breaks.
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Only after the wiring, adjustment and stopper pad have been ruled out, test the switch itself for correct opening and closing of both contact sets through the full pedal travel.
Related powertrain codes
Frequently asked questions
Can I keep driving with P0504?
Treat this as urgent rather than routine. The fault sits in the same circuit as your brake lights, so drivers behind you may get no warning that you are slowing, and on some vehicles you will not be able to shift out of Park at all. Check that your brake lights work before driving, and get the fault fixed quickly even if the car otherwise seems fine.
Will this fail an emissions test?
Yes, in most OBD-based inspection programmes, because the check engine light being on is an automatic fail regardless of what set it. In areas with a safety inspection, non-working brake lights would fail that part of the test as well.
Is this expensive to fix?
Usually not. The brake pedal switch is an inexpensive part and is normally accessible from under the dashboard, and on many vehicles the real culprit is a small plastic stopper pad on the pedal bracket that costs very little. Costs only climb if the fault turns out to be damaged wiring behind the dash or a module problem.
My brake lights work fine, so why is this code set?
The code is about the two brake switch circuits disagreeing with each other, not about the lights failing. One set of contacts can switch late, early or intermittently while the other still operates the lamps normally. That is enough for the ECU to fail its plausibility check and store P0504.
Disabling P0504 in software
RaceTune can permanently disable P0504 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
ECU families we can disable P0504 on
We hold the DaVinci A2L disable definitions for these families, so the exact P0504 path and mask addresses are mapped. verified marks a confirmed disable definition. We support many more — upload your file and our identifier will match it automatically.
- Bosch EDC17C60 verified
- Bosch MG1CP002 verified
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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