Fix P0504: Mechanic’s solutions that WORKED

The P0504: Brake Switch Circuit ‘A’ / ‘B’ Correlation indicates a discrepancy between two parts of the brake light switch circuit, suggesting a problem in how the brake system’s signals are being processed or relayed. This can impact crucial safety functions like brake lights and potentially other systems related to braking.

P0504 quick overview

Meaning	P0504: Brake Switch Circuit ‘A’ / ‘B’ Correlation
Is it serious?	This code can be serious as it relates to the vehicle’s braking system, potentially affecting driving safety.
Possible causes	– Faulty or malfunctioning brake light switch – Damaged or corroded wiring and connectors – Short circuit in the brake switch circuit – Issues with the vehicle’s Engine Control Module (ECM)
How to diagnose?	Inspect Brake Light Switch: Check for any visible damage or signs of malfunction. Examine Wiring and Connectors: Look for any damage, corrosion, or loose connections in the brake system’s wiring. Check Brake Switch Signal: Use a diagnostic scanner to monitor the brake switch signal for proper operation. Test for Short Circuits: Perform electrical tests to identify any shorts in the circuit. Verify ECM Data: Ensure the ECM is correctly interpreting brake system signals. Identify Aftermarket Modifications: Check for any aftermarket installations that might interfere with the brake system. Perform Voltage Tests: Conduct voltage tests on the brake circuit for proper electrical functioning. Consider Related Systems: Investigate related systems like ABS that could be affected by or contributing to the fault.

How to fix P0504 – learn from mechanics

I looked into P0504 service cases at iatn.com website where in the private forums the mechanics seek advice from other mechanics. I looked up a bunch of cases to help you decide how to fix your issue and see what would a real mechanic do to solve it.

Case #1: Toyota

Vehicle Information: 2004 Toyota 4Runner Sport 4.0L Transfer Case

Issue + Repair History:

• 4WD inoperative, with green and amber lights flashing when engaging switch to 4 high.
• No noise from actuator at transfer case.
• Suspected issues with the transfer case actuator.
• Recent discovery of 3 stored codes: P0504 in the engine ECM and C1223 & C1246 in the ABS ECU.

Other OBD2 Codes + Test Results:

• P0504 (Brake switch problem)
• C1223
• C1246

Suggestions to Fix Issue:

• Check the actuator in the front differential, as it may stick if not used for a long time.
• If the actuator has power, consider that the transfer case might need to be removed and torn down.
• Investigate if the vehicle shifted into four-wheel-drive while the rear wheels were spinning, potentially breaking the front differential actuator.
• Consider replacing the whole transfer case with a used unit due to the high labor hours required for actuator replacement.
• Tap the actuator motor with a small hammer while switching the 4WD button on and off.
• Check pin connections at front and transfer actuators, and the ECU for 4WD.
• Inspect the electrical connector near the front differential for corrosion or broken wiring.
• Explore the possibility of replacing the ABS ECU as it interlinks with 4WD operation.
• Clean out and regrease the transfer case and front diff actuators if the grease has hardened.

Final Fix:

• Started with addressing the P0504 brake switch code, which seemed to be setting ABS codes.
• Replaced the brake switch with a factory part after intermittent loss of brake switch signal was noticed.
• Cleared codes and after extended test driving, no codes returned.
• 4WD still inoperative; customer decided to hold off on further 4WD repairs.
• Possible related parts for 4WD repair were very expensive.

Case #2: Land Rover

Vehicle Information: 2007 Land Rover Range Rover Sport Supercharged 4.2L

Issue + Repair History:

• Had a P0504 code (brake switch correlation).
• Issues with “3 Amigos” lights (common term for three warning lights appearing together on the dashboard, often related to ABS, traction control, and hill descent).
• Passenger rear park lamp had no voltage, but brake lamp worked.
• Front air shock seemed lower than others.
• Recent repair history includes brake switch replacement, new pads, rotors, and ball joints.

Other OBD2 Codes + Test Results:

• P0504 – Brake switch correlation

Suggestions to Fix Issue:

• Check for wiring issues, particularly at the rear harness.
• Ensure the rear hatch is locked properly to avoid false signals of being open.
• Investigate the possibility of an ABS pump fault.
• Check if wheel bearings, particularly the left front, have too much play, which could trigger the warning lights.

Final Fix:

• The park light issue was resolved due to a wiring problem at the rear harness.
• The rear hatch issue was due to it not being locked properly; it functioned normally after being properly closed.
• The “3 Amigos” issue was suspected to be an ABS pump fault, but the customer declined repairs.
• Replacing brake switch fixed the P0504

Case #3: Chevrolet

Vehicle Information: 2006 Chevrolet Silverado 3500 WT 6.6L

Issue + Repair History:

• Cruise control inoperative.
• Idle up feature for the PTO was inoperative.
• Technician checked with Tech 2: cruise switch did not change, but brake light did.
• PCM had a P0504 set.
• Checked power and grounds to brake switch; all OK.
• Replaced brake pedal cruise switch; no change.
• Tested circuit #420 for short to power.
• Installed new ECM; still no change at scanner.
• Related to the cruise control being inoperative and affecting the PTO idle up feature.

Other OBD2 Codes + Test Results:

• P0504 – Brake switch circuit

Suggestions to Fix Issue:

• Check for wiring issues, especially if trailer wiring is present.
• Consider if the switch is adjusted correctly or if bypassing the switch helps.
• Inspect the clutch pedal switch.

Final Fix:

• Found an open circuit from the brake switch to the PCM that was missed initially.
• The PCM looks for an absence of voltage to change the input signal.
• Replaced the brake switch with a factory part.
• After clearing codes and retesting, the cruise control and PTO idle up feature returned to normal operation.

Case #4: Chevrolet

Vehicle Information: 2007 Chevrolet Express 3500 6.6L

Issue + Repair History:

• Vehicle not going into regeneration mode.
• Trouble codes P2463, P244B, and P0504 were stored.
• Regeneration attempted with Tech II, but system wouldn’t enter regeneration.
• Brake light switch replaced.
• No stop light switch data in ECM data lists; found in ABS section only, showing “on” regardless of pedal position.
• Vehicle has an idle speed up conversion (aftermarket).

Other OBD2 Codes + Test Results:

• P0504: Brake switch correlation
• P2463: Particulate filter
• P244b: Particulate filter pressure too high

Suggestions to Fix Issue:

• Check if there are two separate issues: a faulty brake switch and a clogged Diesel Particulate Filter (DPF).
• Explore reprogramming options for DPF concerns and ensure VIN in modules is correct.
• Ensure the exhaust hasn’t been modified; suggest using exhaust fluid to reduce soot.
• Consider that the DPF may be plugged and can be cleaned by a radiator shop.
• Check brake switch setup in Tech II under ECM module setup function.
• For 2006 models, check brake switch parameters in Tech II under cruise data.
• Ensure the Center High Mounted Stop Lamp filament is functioning properly.
• Check for possible interference from aftermarket installations or modifications.

Final Fix:

• Found that the wiring tied into the brake switch from the aftermarket idle up system was shorted, causing the computer to see the brake switch as “on” all the time.
• Found the issue in the ABS data list.
• Regeneration functioned properly once the brake pedal was recognized as not being constantly engaged.
• Road-tested the vehicle for about 20 miles to verify the repair, and regeneration was done multiple times successfully.

Case #5: Volvo

Vehicle Information: 2003 Volvo C70 2.3L (Correction: 2001 model)

Issue + Repair History:

• MIL Lamp on with a P1550 fault code.
• ABS light occasionally came on but didn’t set an ABS fault.
• Initially, there was uncertainty about the meaning of the P1550 code.

Computer Codes & Descriptions:

• P1550: Fault in the ABS module (ECM-510F).

Suggestions to Fix Issue:

• Consider replacing the Electronic Brake Control Module (EBCM).
• The ABS module (EBCM) is plug and play, with no programming needed.
• Check if the ABS brain failure is common for the 2001 model.
• The ABS brain can be repaired or replaced; a new one from Volvo could be costly.

Final Fix:

• Replaced the ABS module with a rebuilt one from BBM in Massachusetts for $100.
• Plugged in the new module, and the problem was resolved.

Advice	Description of Diagnostic Step
Check/Replace Brake Light Switch	Inspect and, if necessary, replace the brake light switch as it’s often the primary cause of P0504.
Inspect Wiring and Connectors	Examine wiring and connectors related to the brake system for any damage, shorts, or disconnections.
Verify Brake Switch Signal	Use a diagnostic scanner to check the brake switch signal in the vehicle’s data list, ensuring it operates correctly.
Check for Short Circuits	Test the relevant circuits for potential short circuits that might be causing the code.
Confirm Proper ECM Data Reading	Ensure that the Engine Control Module (ECM) is correctly interpreting signals from the brake system.
Look for Aftermarket Modifications	Identify any aftermarket installations or modifications that might interfere with the brake system’s normal operation.
Test Circuit Voltage	Perform voltage tests on the brake circuit to ensure proper electrical functioning.
Consider Related Systems	Investigate related systems (like ABS or cruise control) that could be affected by or contributing to the fault.