EN

Severity: Moderate Code type: Generic (all makes)

P0504 Code: Brake Switch A/B Correlation

The two contacts in the brake pedal switch disagree on whether you're braking; check the pedal stopper first.

Safety first · Consult a professional

  • Braking system affected: ABS may be disabled. Drive carefully and have it checked promptly.

At a glance

Severity: 3/5 – Moderate3/5

Severity: Moderate

Meaning
Brake Switch A/B Correlation
System
Speed & Idle Control · Powertrain
Safe to drive?
With caution
Typical repair cost
$0–$450
DIY difficulty
Easy

Most brake pedal switches contain two separate contacts that work in opposite directions: circuit A closes when you press the pedal (it feeds the brake lights and a computer input), circuit B opens at the same moment (it confirms the pedal for cruise control and the engine computer). The computer expects one to be on whenever the other is off. P0504 sets when both read the same for too long, for example both on or both off while you drive. The usual reasons are mechanical: a crumbled rubber or plastic stopper pad on the pedal arm, a switch that has slipped out of adjustment, or one of the two feed fuses blown. Because the brake signal also controls the shift interlock, push-button starting and stability control, the side effects can be more annoying than the check engine light.

Can I keep driving? Check your brake lights before driving any distance; if they stay on constantly or don't come on at all, fix that first. Otherwise the car is drivable, but cruise control, start-stop and sometimes stability control will be disabled.

Try this first – 5-minute fix

  1. Replace the pedal stopper pad If the pad is missing, fit a new one (a very cheap part) so the switch plunger presses on a solid surface again. Then check that the brake lights go out with the pedal released.
  2. Re-seat and adjust the switch Push the self-adjusting switch in with the pedal pressed, then pull the pedal up by hand so it sets itself, or set the specified gap on threaded types. Both contacts should now change within the first bit of pedal travel.
  3. Check fuses and clear the code Replace any blown stop-lamp or ignition fuse, clear P0504 and test-drive with several stops to confirm cruise control works again.

How to read P0504

How to read P0504P10253044
  1. P Powertrain
  2. 0 Generic (SAE)
  3. 5 Speed, idle & inputs
  4. 04 Specific fault

Symptoms

  • Brake lights stay on with the pedal released, or don't come on at all
  • Cruise control won't engage or cancels at random
  • Shifter won't come out of Park, or a push-button start car won't crank
  • Stability control, traction or ABS warning lights come on together with the check engine light
  • Dead battery overnight because brake lights stayed on

Possible causes

Ranked by how often each cause is behind this code (editorial estimate).

  1. Stopper pad on the pedal arm worn or missing, so the switch plunger goes through the hole and one contact never resets 30%
  2. Switch out of adjustment after pedal, dash or switch work 22%
  3. Internal wear in one of the two switch contacts 20%
  4. Blown fuse feeding only one of the two circuits 12%
  5. Backfeed from trailer wiring or poorly installed LED brake bulbs holding circuit A on 10%
  6. Wiring or computer input fault 6%

Diagnosis step by step

Diagnostic flow: P05041Check the brake lights both ways2Look at the pedal arm3Compare A and B on live data4Check both fuses5Measure the switch6Rule out backfeed

  1. Check the brake lights both ways

    Look at the brake lights with the pedal released and pressed; use a wall reflection if you're alone. Lights stuck on point to circuit A being held closed (missing stopper, backfeed); lights never on point to a blown stop-lamp fuse or a switch that isn't reaching the pedal.

    Tools: Flashlight

  2. Look at the pedal arm

    Lie in the footwell with a light and look where the switch plunger touches the pedal. Many vehicles use a small rubber or plastic bumper there. If it has crumbled, you'll see the plunger sitting in an empty hole and pieces on the carpet.

    Tools: Flashlight

  3. Compare A and B on live data

    On a scan tool, show both brake switch inputs. Pedal up, one should read off and the other on; pedal down, they should swap within a fraction of an inch of travel. If both read the same in either position, note which one is wrong.

    Tools: OBD-II scanner

  4. Check both fuses

    Circuit A is often fed from a constant stop-lamp fuse, circuit B from an ignition-switched fuse. Check both with a test light or meter; a blown one leaves one circuit dead and sets P0504 immediately.

    Tools: Multimeter

  5. Measure the switch

    Unplug the switch and check each pair of pins with an ohmmeter: one pair open with the plunger out and closed when pushed in, the other pair the opposite. Both should switch cleanly with no flicker as you move the plunger slowly.

    Tools: Multimeter

  6. Rule out backfeed

    If circuit A reads on with the pedal up, unplug the trailer connector and any add-on lighting modules. If the reading drops to off, the trailer wiring or an LED conversion is feeding voltage back into the brake light circuit.

    Tools: Multimeter, OBD-II scanner

Where the parts are

Ignition: coils and spark plugs per cylinder – P05041234567
Ignition: coils and spark plugs per cylinder
  1. 1Ignition coil
  2. 2Spark plug
  3. 3Fuel injector
  4. 4Camshaft sensor
  5. 5Crankshaft sensor
  6. 6Engine computer (ECM)
  7. 7Intake manifold

Repair options & costs

RepairDIY?Estimated cost
Replace brake pedal stopper pad Yes $5–$60
Adjust brake light switch Yes $0–$80
Replace brake light switch Yes $40–$180
Repair trailer wiring or remove backfeeding LED/lighting module Yes $30–$200
Repair wiring or computer input No $100–$450

Rough US estimates for parts and labor; shop rates vary by region. Not a quote.

Notes by brand

Hyundai

Hyundai and Kia models are well known for the brake pedal stopper pad disintegrating with age. The switch plunger then drops into the empty hole, the brake lights may stay on, and codes such as P0504 appear along with a shifter that won't leave Park.

P0504 on Hyundai in detail

Field data

Hyundai 8

Source: NHTSA Complaints API (US government open data), analyzed by DTC mention.

Emissions / smog test impact

Depends

Many vehicles don't turn on the check engine light for P0504, so it may not affect an OBD emissions test. Where the light is on, the test fails, and brake lights that stay on or don't work are a safety inspection failure in states that do one.

Frequently asked questions

How is P0504 different from P0571?

P0571 flags the main brake switch circuit as implausible or intermittent. P0504 specifically means the two switch circuits disagree with each other, which often has a simple mechanical cause like a missing stopper pad or a switch that's out of adjustment.

Why can't I shift out of Park?

The shift interlock releases only when the computer sees the brake pedal pressed. If the switch circuits are confused, the interlock may stay locked. Most vehicles have a manual release slot near the shifter for emergencies; check your owner's manual.

Can a trailer cause P0504?

Yes. A trailer or a towing module with a short, or cheap LED bulbs without proper load resistors, can feed voltage back into the brake light circuit. The computer then sees the brake pedal pressed while circuit B says it isn't.

Is it safe to drive with P0504?

The brakes themselves work normally. The risk is to others: brake lights that never come on or never go off mislead drivers behind you. Fix that part right away; the rest can wait for a convenient repair.

Sources