OBD-II Diagnostic Trouble Code
P0504 — Brake Switch A/B Correlation
P0504 means the two brake switch signals (A and B) disagree. Most trucks use two brake inputs, one normally open and one normally closed, and the ECM expects them to change state together; it saw one switch change without the other.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Brake Switch A/B Correlation
Code
P0504
Category
Powertrain
Subsystem
Speed
Truck-relevant
Yes
What this means for truckers
Service within a few days on a Cummins X15, Detroit DD15, PACCAR MX-13. Performance is likely degraded. Continuing to drive without service risks additional component damage.
Before you replace parts — try this first
P0504 on the Speed system is most often misadjusted brake switch — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Cruise control may not engage or may cancel; brake lights may stay on or not work; on some trucks exhaust brake or engine brake functions and idle shutdown behave incorrectly. On automatics, shift interlock may be affected.
Common causes
- CommonMisadjusted brake switch ($50–$150)
- CommonFailed brake switch (one contact set) ($80–$250)
- OccasionalWiring or connector fault at the switch ($100–$300)
- OccasionalBlown brake light fuse or bulb affecting one circuit ($20–$100)
How to fix
Watch both brake switch states on a scan tool while pressing the pedal
When to use: To see which input is not changing
Adjust the switch per OEM procedure
When to use: When both circuits work but change at different points
Replace the brake switch
When to use: When one circuit does not change state
Check fuses and wiring for the brake light and brake input circuits
When to use: When the switch tests good
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P0500 — Vehicle Speed Sensor Malfunction
- → P0501 — Vehicle Speed Sensor Range/Performance
- → P0502 — Vehicle Speed Sensor Low Input
- → P0503 — Vehicle Speed Sensor Intermittent/Erratic/High
- → P0506 — Idle Control System RPM Lower Than Expected
Frequently Asked Questions
Can I drive with code P0504?
P0504 is severity 3 — plan service within a few days. Performance is likely degraded already (you may feel reduced power, harsher shifts, or slightly worse fuel economy). Continuing to drive risks additional damage to adjacent components.
How much does it cost to fix P0504 on a commercial truck?
Total repair cost ranges $20–$300 depending on the root cause. The cheapest fix is usually the first one listed below — Misadjusted brake switch — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P0504 fail a DOT inspection?
P0504 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P0504 and related codes?
P0504 is closely related to: P0500 (Vehicle Speed Sensor Malfunction), P0501 (Vehicle Speed Sensor Range/Performance), P0502 (Vehicle Speed Sensor Low Input), P0503 (Vehicle Speed Sensor Intermittent/Erratic/High). These codes share the same subsystem (speed) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P0504 is set without the companions, the issue is more isolated.
How do I prevent P0504 from coming back after a repair?
For P0504 (Brake Switch A/B Correlation), prevention centers on the speed system. Inspect wheel speed sensor wiring at PM (rodents love it). Keep ABS tone rings clean.