OBD-II trouble code
P0504: Brake Switch "A"/"B" Correlation
The two signals from the brake pedal switch disagree with each other — the computer can't tell for sure whether you're pressing the brake. Usually the switch itself.
Quick facts
- System
- Powertrain
- Category
- Electrical / PCM
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $30 – $250
- DIY difficulty
- Beginner DIY
What does P0504 mean?
Modern vehicles use a brake pedal switch with two separate circuits ("A" and "B") that change state in opposite or offset ways when you press the pedal. That redundancy matters because the brake signal does a lot: it lights the brake lamps, releases the shift interlock, cancels cruise control, and on many vehicles informs stability control and push-button start. P0504 sets when the two signals disagree — one says the pedal is pressed while the other says it isn't — so the computer can't trust either.
The overwhelmingly common cause is the brake switch itself: internal contacts wear, misalign, or one circuit becomes intermittent. Misadjustment after pedal or switch service is the next suspect — many switches must be adjusted so both circuits trip in the correct sequence. Wiring damage at the pedal-area connector, or a low-voltage condition briefly corrupting one channel, cover most remaining cases.
Symptoms are more everyday-annoying than dramatic: cruise control refusing to engage, a shifter that occasionally won't come out of Park, brake lights acting up, or a harsh downshift feel on vehicles that use the brake signal for transmission strategy.
Common causes
- Worn or failing brake pedal switch (one circuit intermittent)
- Misadjusted brake switch after service
- Damaged wiring or connector at the pedal switch
- Corroded terminals in the switch connector
- Low system voltage briefly corrupting one signal
Symptoms
- Check engine light on
- Cruise control won't engage or randomly cancels
- Shifter intermittently won't release from Park
- Brake lights stuck on or not lighting
- Harsh or odd transmission behavior on some vehicles
Diagnostic steps
- 1.Watch both brake switch signals on live data while slowly pressing and releasing the pedal — they should change state cleanly and in the correct relationship.
- 2.Check brake light operation, including with slight pedal pressure where correlation faults often show.
- 3.Inspect the switch connector and wiring above the pedal for damage or looseness.
- 4.Verify switch adjustment against the service procedure.
- 5.Replace the switch (inexpensive) if signals disagree with wiring good; adjust per spec.
- 6.Clear codes and confirm cruise, shift interlock, and brake lights all behave.
Repair cost
$30 – $250
A brake light switch is a $15-$60 part and typically a 15-45 minute job at the top of the pedal — one of the cheaper fixes in the book. Higher costs only appear if wiring repair is needed or a pedal-area trim fight is involved.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with brake light switch replacement preselected. Adjust labor rate and vehicle category to fit your situation.
Related repairs
DIY vs shop
This is a beginner-friendly repair. Common hand tools, a free afternoon, and a willingness to follow a procedure are usually enough. The risk of causing a bigger problem is low if you read up on your specific vehicle first.