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DTC codes / P05xx · Vehicle speed, idle control and auxiliary inputs / P0504

P0504Brake Switch 'A'/'B' Correlation

P0504 means the powertrain control module (PCM) has detected a disagreement between the two electrical signals produced by the brake pedal switch assembly, which typically contains a normally closed and a normally open contact. The PCM compares these two inputs to confirm the brake pedal position, and when the signals do not correlate for a calibrated period, it stores this code and may disable cruise control or affect shift logic.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaVehicle speed, idle control and auxiliary inputs

Symptoms you may notice

  • The brake warning light or check engine light illuminates on the dashboard.
  • Often no symptoms besides the warning light, with the vehicle driving normally.
  • Cruise control may fail to engage or may disengage unexpectedly when the brake pedal is touched.
  • The transmission may exhibit harsh or delayed shifting because the PCM cannot confirm brake pedal status.
  • Brake lights may stay on continuously or fail to illuminate when the pedal is pressed.
  • Some vehicles may refuse to start or may not release the shift interlock from Park.

Common causes, most likely first

  1. A faulty brake pedal switch assembly where the internal contacts have worn or become contaminated.
  2. Misadjusted or loose brake switch mounting, causing the two contacts to change state at different pedal positions.
  3. Damaged, corroded, or chafed wiring in the brake switch circuits between the switch and the PCM.
  4. A blown fuse or poor ground connection that affects only one of the two brake switch signal circuits.
  5. Brake pedal binding or a missing pedal return bumper that prevents the switch from actuating correctly.
  6. Water intrusion or debris in the switch connector causing intermittent signal correlation errors.
  7. In rare cases, a PCM input circuit fault or software issue that misinterprets the two brake signals.

How to diagnose and fix it

  1. Verify the brake lights operate correctly and check both brake light fuses before touching the switch.
  2. Inspect the brake pedal switch mounting and adjustment, confirming the plunger contacts the pedal arm properly.
  3. Use a scan tool to view live data for both brake switch inputs while slowly pressing and releasing the pedal, looking for a moment where both signals read the same state.
  4. Backprobe the switch connector and check for voltage and ground on both signal circuits with the pedal released and pressed.
  5. Check the wiring harness between the switch and the PCM for chafing, corrosion, or loose pins, especially near the pedal bracket.
  6. Replace the brake pedal switch assembly if the live data confirms the internal contacts do not correlate correctly.
  7. Clear the code and perform a road test with cruise control and several brake applications to confirm the repair.

Why brake switch correlation codes mislead

  • A brake switch that still turns on the brake lights can be internally faulty on the second contact, so a visual brake light check alone does not rule out the switch.
  • Technicians often replace the switch without checking pedal free play or the rubber bumper on the pedal arm, and the new switch sets the same code because the pedal never releases it fully.
  • This code can be set by a failing brake light bulb or socket on some vehicles because the PCM monitors the brake signal circuit through the bulb filament.
  • Intermittent P0504 codes that appear only in cold or wet weather usually point to a connector or harness issue rather than the switch itself.
  • A misadjusted switch can cause the torque converter clutch to stay engaged or the transmission to shift harshly, so the code should not be ignored just because the car drives.

Can I keep driving with P0504?

The vehicle is generally safe to drive to a repair shop because the base brakes remain fully functional, but avoid using cruise control and be aware that brake lights or shift interlock behavior may be affected. If the brake lights stay on continuously, do not leave the vehicle parked unattended for long periods because the battery may drain.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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