What causes fault code P0504?

• Most likely: Faulty brake light switch (internal mechanical or electrical failure) • Likely: Incorrectly adjusted brake switch at the pedal assembly • Possible: Wiring harness damage or short circuit in the brake lighting circuit (e.g., trailer hitch wiring) • Rare: Incorrect bulb types (LED conversion without resistors) or corroded/shorted bulb sockets

How do you fix fault code P0504?

STEP 1: Perform a visual check of the brake light operation and inspect stop-lamp fuses. Check the trailer plug/socket for moisture or oxidation. STEP 2: Use a diagnostic scanner to view Live Data. Monitor 'Brake Switch A' and 'Brake Switch B' status. They must toggle state simultaneously when the pedal is pressed. STEP 3: Perform a continuity test (Ohms) on the switch pins. One circuit should show < 1 Ohm (closed) while the other shows OL (open) during rest, reversing states when pressed. STEP 4: Verify 12V power supply to the switch. If wiring is intact but signals are inconsistent, replace the brake light switch and ensure proper mechanical adjustment at the brake pedal arm.

Powertrain (Engine & Gearbox)Standard EOBD

P0504 - Brake Switch A/B Correlation

Q: What does fault code P0504 mean?

P0504 is set when the engine control unit (ECU) or ABS module detects an inconsistency between the two signals originating from the brake light switch. Modern vehicles use a redundant circuit design: one switch contact closes (Normally Open) to activate brake lights, while the other opens (Normally Closed) to signal the ECU. If the signals do not transition simultaneously or within a calibrated timeframe, the ECU detects a correlation failure. This logic is crucial for safety systems, cruise control inhibition, and brake-transmission shift interlock (BTSI).

Q: What are the symptoms of fault code P0504?

• Check Engine Light (MIL) illuminated • Cruise control is disabled/inoperative • Brake lights stay on constantly or fail to illuminate • Gear shifter cannot be moved out of 'Park' • Vehicle fails inspection due to safety-related lighting/DTC

Last updated:1 February 2026

Related:Connector, ECU / Motorstyreenhet, BCM / Karosseristyremodul, Wiring Harness, Connector

System

Powertrain (Engine & Gearbox)

Related Fault Codes

P0500

Vehicle Speed Sensor (VSS) 'A' Range/Performance

P0501

Vehicle Speed Sensor 'A' Range/Performance

P0502

Vehicle Speed Sensor 'A' Circuit Low Input

P0503

Vehicle Speed Sensor (VSS) Intermittent/Erratic/High

Technical Description

P0504 is set when the engine control unit (ECU) or ABS module detects an inconsistency between the two signals originating from the brake light switch. Modern vehicles use a redundant circuit design: one switch contact closes (Normally Open) to activate brake lights, while the other opens (Normally Closed) to signal the ECU. If the signals do not transition simultaneously or within a calibrated timeframe, the ECU detects a correlation failure. This logic is crucial for safety systems, cruise control inhibition, and brake-transmission shift interlock (BTSI).

Possible Causes

Faulty brake light switch (internal mechanical or electrical failure)

Incorrectly adjusted brake switch at the pedal assembly

Wiring harness damage or short circuit in the brake lighting circuit (e.g., trailer hitch wiring)

Incorrect bulb types (LED conversion without resistors) or corroded/shorted bulb sockets

Symptoms

Check Engine Light (MIL) illuminated

Cruise control is disabled/inoperative

Brake lights stay on constantly or fail to illuminate

Gear shifter cannot be moved out of 'Park'

Vehicle fails inspection due to safety-related lighting/DTC

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P0504 - Brake Switch A/B Correlation

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P0504 - Brake Switch A/B Correlation

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