OBD-II Diagnostic Trouble Code
P0571 — Brake Switch A Circuit
P0571 indicates a fault in the brake switch A circuit. The ECM did not see the brake switch A input change as expected, or found the signal implausible compared with other inputs such as vehicle speed changes.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Brake Switch A Circuit
Code
P0571
Category
Powertrain
Subsystem
Speed
Truck-relevant
Yes
What this means for truckers
P0571 (Brake Switch A Circuit) — the brake pedal position switch is confused. Cruise control disables, transmission may not downshift on braking, brake lights may flicker. Common fix: replace brake pedal switch ($30-80 part, 15-min install). Failure mode is usually contact wear after years of pedal cycling.
Before you replace parts — try this first
P0571 on the Speed system is most often faulty or 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 will not engage or will not cancel with the brake. Engine or exhaust brake may not work correctly, and brake lights may not operate if they share the circuit.
Common causes
- CommonFaulty or misadjusted brake switch ($80–$250)
- OccasionalWiring or connector fault ($100–$300)
- OccasionalBlown fuse ($20–$100)
- OccasionalAir brake pressure switch fault on air-braked trucks ($80–$250)
How to fix
Monitor brake switch status on a scan tool while applying the brake
When to use: First step
Adjust or replace the brake switch
When to use: When status does not change
Check fuses and wiring from switch to ECM
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 P0571?
P0571 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 P0571 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 — Faulty or 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 P0571 fail a DOT inspection?
P0571 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 P0571 and related codes?
P0571 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 P0571 is set without the companions, the issue is more isolated.
How do I prevent P0571 from coming back after a repair?
For P0571 (Brake Switch A Circuit), prevention centers on the speed system. Inspect wheel speed sensor wiring at PM (rodents love it). Keep ABS tone rings clean.