P0504: How to Diagnose & Fix — Brake Switch "A"/"B" Correlation
6 vehicles · 4 makes · $2-$250 repair
How to diagnose and fix P0504: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
Ford Mustang · 2015–2018
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)
Chevrolet Colorado · 2004–2012
The brake lamp switch at the pedal is a known weak point on first-generation Colorado trucks. Owners report brake lights staying on, not coming on reliably, inability to shift out of park, or cruise control dropping out unexpectedly. Because the switch feeds several circuits, a single failure can create multiple confusing symptoms.
Common Symptoms
- brake lights stay on
- brake lights do not illuminate consistently
- cannot shift out of park
- cruise control inoperative
- battery drains overnight from brake lamps staying on
How to Fix
Inspect brake lamp operation first, then verify switch adjustment and continuity at the pedal assembly. Replacement of the brake light switch is usually straightforward and inexpensive, but technicians should also check for damaged pedal bushings or connector issues if symptoms persist. After replacement, confirm proper brake lamp operation and shift interlock release.
Sources for this issue (1)
Kia Optima · 2008–2010
Kia recall SC098 (NHTSA 13V-114) covers 2008-2011 Optima vehicles with a stop-lamp switch that may malfunction. Brake lamps may fail to illuminate, cruise control may not disengage, push-button start or brake-transmission shift interlock operation may be affected, and the ESC warning light may illuminate. This indexed page retains its existing 2008-2010 year scope.
Common Symptoms
- Brake lamps do not illuminate
- Cruise control does not disengage with brake pedal
- Shift interlock or push-button start operates intermittently
- ESC warning light illuminated
How to Fix
Check the VIN for open recall SC098/13V-114 and have the free dealer repair completed. Kia dealers replace the stop-lamp switch. Because recall applicability is VIN-specific and the campaign remedy is free, no retail switch is linked.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (1)
Kia Sedona · 2007–2011
Chevrolet Malibu · 2004–2012
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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?
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.
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?
Repair costs for P0504 range from $2 to $250, depending on the vehicle and root cause.
Which vehicles are affected by P0504?
Au7o has documented P0504 across 6 vehicle models from 4 manufacturers: Chevrolet, Ford, Kia, Nissan.
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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.