Worn or failed brake light switch; OBD-Codes.com calls it the most common cause (both sources). Switch out of adjustment so it no longer contacts the pedal arm properly (OBD-Codes.com). Wiring or connector problems, such as loose connectors or pushed-out pins (both sources). Blown brake light fuse, sometimes from moisture in the circuit (OBD-Codes.com). Failed brake light bulb, often from water getting into the lens (OBD-Codes.com). Faulty PCM, which is uncommon (both sources).

Replace the brake light switch (the most common fix per both sources). Adjust the brake light switch if it is the adjustable type (OBD-Codes.com). Replace a blown fuse or failed bulbs (OBD-Codes.com). Repair damaged wiring or connectors between the switch and the PCM (both sources). PCM repair or replacement only after the switch and wiring are ruled out (both sources list it as a possible cause).

P0504🔧 Brake light switch / brake pedal signalStop and fix promptly

P0504: Brake Switch A/B Correlation (Meaning, Causes, Fixes)

Q: What does P0504 mean?

P0504 means the engine computer sees the two signals from the brake light switch disagreeing with each other. The most common cause is a worn brake light switch, and typical symptoms are brake lights that don't work or stay on, cruise control that won't engage, and trouble shifting out of park. The brake light switch sits under the dash at the top of the brake pedal arm. RepairPal explains that it tells the powertrain control module (PCM) when the brake pedal is pressed and also turns on the brake lights. OBD-Codes.com notes the switch has two signal outputs (the 'A' and 'B' circuits); the PCM expects both to change together when you press the pedal. If they don't agree, or the PCM sees an incorrect signal, it sets P0504. Besides the brake lights, the brake pedal signal is used for cruise control cancel and, on many vehicles, the shift interlock that lets you move the shifter out of park. Both sources agree on the definition and that the switch is the most likely cause. They differ in how wide they cast the net: RepairPal lists only the switch, wiring and the PCM, while OBD-Codes.com also lists a blown brake light fuse and failed brake light bulbs as possible causes.

Q: Can I keep driving with P0504?

Stop and fix promptly. RepairPal rates P0504 urgent, and OBD-Codes.com says the safety factor demands a repair as soon as possible. The brake light switch tells following drivers you are stopping, so if your brake lights stay dark (or stay on all the time) you are at higher risk of being rear-ended. OBD-Codes.com adds that it can also make it hard to shift out of park. Check your brake lights right away and get the switch fixed promptly; both sources say it is usually not an expensive repair.

Reviewed 2026-10-22 · Based on 2 cited source(s)

What P0504 means

The brake light switch sits under the dash at the top of the brake pedal arm. RepairPal explains that it tells the powertrain control module (PCM) when the brake pedal is pressed and also turns on the brake lights. OBD-Codes.com notes the switch has two signal outputs (the 'A' and 'B' circuits); the PCM expects both to change together when you press the pedal. If they don't agree, or the PCM sees an incorrect signal, it sets P0504. Besides the brake lights, the brake pedal signal is used for cruise control cancel and, on many vehicles, the shift interlock that lets you move the shifter out of park. Both sources agree on the definition and that the switch is the most likely cause. They differ in how wide they cast the net: RepairPal lists only the switch, wiring and the PCM, while OBD-Codes.com also lists a blown brake light fuse and failed brake light bulbs as possible causes.

Severity: can I keep driving?

Stop and fix promptly

RepairPal rates P0504 urgent, and OBD-Codes.com says the safety factor demands a repair as soon as possible. The brake light switch tells following drivers you are stopping, so if your brake lights stay dark (or stay on all the time) you are at higher risk of being rear-ended. OBD-Codes.com adds that it can also make it hard to shift out of park. Check your brake lights right away and get the switch fixed promptly; both sources say it is usually not an expensive repair.

Common symptoms

Check engine light on
Brake lights don't come on when you press the pedal, or stay on all the time (both sources)
Cruise control won't engage (both sources)
Hard or impossible to shift out of park (OBD-Codes.com)
Vehicle may stall when the brakes are applied at cruising speed (OBD-Codes.com)

Likely causes

Roughly most to least common, based on the sources below. Your vehicle may differ.

Worn or failed brake light switch; OBD-Codes.com calls it the most common cause (both sources)
Switch out of adjustment so it no longer contacts the pedal arm properly (OBD-Codes.com)
Wiring or connector problems, such as loose connectors or pushed-out pins (both sources)
Blown brake light fuse, sometimes from moisture in the circuit (OBD-Codes.com)
Failed brake light bulb, often from water getting into the lens (OBD-Codes.com)
Faulty PCM, which is uncommon (both sources)

Diagnostic steps you can do

⚠ Before you start

Work on a cool engine, parked on level ground with the parking brake set. Never work under a car supported only by a jack.

Have a helper stand behind the car while you press and release the brake pedal, and confirm all brake lights turn on and off.
Replace any burned-out brake light bulbs and check the brake light fuse.
Look up under the dash at the top of the brake pedal arm: make sure the switch connector is fully seated and the switch plunger touches the pedal arm when the pedal is released.
If your switch is the adjustable type, check its adjustment against your service manual. OBD-Codes.com notes some vehicles have a knee airbag near the pedals, so work carefully there.

Typical fixes

Replace the brake light switch (the most common fix per both sources)
Adjust the brake light switch if it is the adjustable type (OBD-Codes.com)
Replace a blown fuse or failed bulbs (OBD-Codes.com)
Repair damaged wiring or connectors between the switch and the PCM (both sources)
PCM repair or replacement only after the switch and wiring are ruled out (both sources list it as a possible cause)

Repair costs vary widely by vehicle, region, parts brand and shop, so we don't quote prices. Get a written estimate after diagnosis.

🛠 When to call a pro

Testing each switch circuit with a voltmeter: OBD-Codes.com describes checking for 12 volts supply at the switch, then back-probing the output wires to confirm they switch on only when the pedal is pressed
If the switch tests good, checking the brake signal at the PCM's input terminal to tell a wiring fault from a computer fault
Working near a driver knee airbag, which OBD-Codes.com warns about

Where to buy

Plain search links. Always match parts to your exact year, make, model and engine.

OBD2 scanners on Amazon Parts at Advance Auto Parts catalog at RockAuto

FAQs

Can I drive with P0504?

Only with care, and only after you have confirmed your brake lights work. RepairPal rates P0504 urgent, and if your brake lights don't come on, drivers behind you won't know you're stopping. Get the switch fixed promptly.

Is P0504 expensive to fix?

Usually not. OBD-Codes.com says it is not an expensive proposition in parts or labor, since the usual fix is a brake light switch under the dash. We don't list a dollar figure because neither source gives one.

Why won't my cruise control work with P0504?

Cruise control relies on the brake switch signal to know when to cancel. When the PCM sees the two brake switch signals disagree, it won't let cruise engage, as both sources note.

Sources

Last reviewed 2026-10-22. Code definitions and repair steps can vary by manufacturer. Confirm with your vehicle's service information.