P0504 on Ford: How to Diagnose & Fix
Brake Switch "A"/"B" Correlation
1 Ford model · 1 documented issue · $2-$250 repair
P0504 on Ford by model: symptoms, diagnosis and fix
A small plastic clip called the brake pedal bumper/stopper can break and fall out of position, causing the brake light switch to remain constantly engaged. This results in brake lights staying on even when the pedal is released (including when the car is off, draining the battery), the engine going into limp mode limiting RPM to around 2,000, and transmission shifting issues.
Common Symptoms
- Brake lights stay on constantly
- Brake lights on even with car off
- Engine won't rev past 2000 RPM
- Limp mode / reduced power
- Dead battery
- Cruise control won't engage
How to Fix
Replace the brake pedal bumper/stopper clip. On 2015 Mustangs covered by recall 22S02 (NHTSA 22V011), Ford dealers replace the brake and clutch pedal bumpers free of charge. Check your VIN at nhtsa.gov/recalls. The part costs $2-10 and can be installed in minutes without tools. Simply press the new bumper into the hole on the brake pedal bracket where the old one fell out. If the battery was deeply discharged, it may need to be replaced ($150-250).
Sources for this issue (3)
P0504 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0504 mean on a Ford?
P0504 stands for "Brake Switch "A"/"B" Correlation." Most vehicles use two independent brake switch signals (a normally-open and a normally-closed circuit, switch A and switch B). The PCM compares them and expects them to change state together when the brake pedal is pressed. P0504 sets when the two signals disagree (do not correlate), indicating one switch or circuit is faulty. On Ford it is documented across 1 model: Mustang.
What are the most common causes of P0504?
The most common causes of P0504 are: Failed brake light switch, Misadjusted brake switch (plunger gap), Wiring/connector fault on one of the two brake switch circuits, Blown brake-circuit fuse affecting one signal, Faulty PCM input. The specific cause varies by vehicle.
How much does it cost to fix P0504 on a Ford?
Published repair estimates on Ford range from $2 to $250 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.