Brake Switch "A"/"B" Correlation
P0504 indicates the brake switch signals (A and B circuits) are not correlating or matching as expected by the PCM. This code sets when the two redundant brake switch signals disagree, suggesting a brake switch malfunction, wiring issue, or connector problem that affects brake pedal detection.
What You Might Notice
- Brake lights stay on continuously or fail to illuminate when pedal is pressed
- Difficulty or inability to shift out of Park
- Vehicle stalls when brakes are applied while cruising
- Cruise control will not activate
- Check Engine light illuminated
Most Common Causes
- 1
Worn or failed brake light switch
The brake switch degrades over time due to wear and contact corrosion, causing signal mismatch between its A and B circuits.
- 2
Loose or corroded switch connectors
Connector pins can become loose, pushed out, or corroded, breaking the electrical circuit and preventing proper signal transmission.
- 3
Damaged brake light wiring harness
Broken, pinched, or damaged wiring between the switch and PCM creates intermittent or missing signals that fail correlation checks.
- 4
Blown brake light fuse
Moisture in the circuit or an electrical overload can burn out the brake light fuse, cutting power to the switch circuit.
How to Diagnose It
- 1
Check Worn or failed brake light switch — The brake switch degrades over time due to wear and contact corrosion, causing signal mismatch between its A and B circuits.
- 2
Check Loose or corroded switch connectors — Connector pins can become loose, pushed out, or corroded, breaking the electrical circuit and preventing proper signal transmission.
- 3
Check Damaged brake light wiring harness — Broken, pinched, or damaged wiring between the switch and PCM creates intermittent or missing signals that fail correlation checks.
- 4
Check Blown brake light fuse — Moisture in the circuit or an electrical overload can burn out the brake light fuse, cutting power to the switch circuit.
How to Fix It
- 1
Replace brake light switch
Remove the old switch from under the dashboard near the brake pedal and install a new OEM or quality replacement, then test for code clearance.
- 2
Inspect and reseat brake switch connectors
Check the connector at the brake switch for loose pins, corrosion, or damaged contacts. Clean contacts with contact cleaner and firmly reseat, or replace connector if damaged.
- 3
Inspect brake light wiring harness
Trace the wiring from brake switch to PCM, looking for pinches, cuts, exposed wires, or water damage. Repair damaged sections or replace entire harness if necessary.
- 4
Check and replace brake light fuse
Locate the brake light fuse in the fuse box, inspect for burn marks or corrosion, and replace with the correct amperage rating if blown. Dry the fuse box if moisture is present.
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