OBD-II Diagnostic Trouble Code
C0091 — Brake Switch B Failure
C0091 indicates a fault in brake switch B, the second brake switch or second circuit of a dual-circuit switch. The module compares switch A and switch B and set the code because switch B was missing, stuck or did not agree with switch A. Exact definitions vary by manufacturer.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
Brake Switch B Failure
Code
C0091
Category
Chassis
Subsystem
Brakes
Truck-relevant
Yes
What this means for truckers
C0091 (Brake Switch B Failure) — secondary brake switch failed.
Before you replace parts — try this first
C0091 on the Brakes system is most often faulty brake switch (b circuit) — 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
The ABS or warning lamp may be on and cruise control may not engage or cancel properly. Brake lights usually still work if switch A is fine.
Common causes
- CommonFaulty brake switch (B circuit) ($30–$150)
- CommonSwitch out of adjustment so A and B change at different points ($30–$100)
- OccasionalWiring fault on the B circuit ($80–$300)
How to fix
Watch switch A and B status on a scan tool while applying the brakes
When to use: First step
Adjust or replace the switch
When to use: When B does not change with A
Check the B circuit wiring for opens or shorts
When to use: When a new switch does not fix it
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
- → C0035 — Left Front Wheel Speed Sensor Circuit
- → C0036 — Left Front Wheel Speed Sensor Circuit Range/Performance
- → C0040 — Right Front Wheel Speed Sensor Circuit
- → C0041 — Right Front Wheel Speed Sensor Circuit Range/Performance
- → C0045 — Left Rear Wheel Speed Sensor Circuit
Frequently Asked Questions
Can I drive with code C0091?
C0091 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 C0091 on a commercial truck?
Total repair cost ranges $30–$300 depending on the root cause. The cheapest fix is usually the first one listed below — Faulty brake switch (B circuit) — 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 C0091 fail a DOT inspection?
C0091 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 C0091 and related codes?
C0091 is closely related to: C0035 (Left Front Wheel Speed Sensor Circuit), C0036 (Left Front Wheel Speed Sensor Circuit Range/Performance), C0040 (Right Front Wheel Speed Sensor Circuit), C0041 (Right Front Wheel Speed Sensor Circuit Range/Performance). These codes share the same subsystem (brakes) 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 C0091 is set without the companions, the issue is more isolated.
How do I prevent C0091 from coming back after a repair?
For C0091 (Brake Switch B Failure), prevention centers on the brakes system. Clean wheel speed sensors at every brake service. Inspect tone rings for cracks or rust. Keep ABS sensor harnesses away from chafe points. Don't drive through deep water that can flood the rear wheel-speed sensor connectors.