Speed Wheel Sensor All Coherency Failure
All four wheel speed sensors are sending conflicting or invalid signals to the vehicle's stability control system, like four people giving completely different directions at the same time. The ECU can't trust any of the speed data to properly control braking or traction.
The ECM monitors all four wheel speed sensors simultaneously and compares their signals for coherency and logical consistency. It checks that wheel speeds match vehicle dynamics and that sensor readings don't contain noise, dropouts, or conflicting data. When all sensors fail coherency checks together, the system cannot establish reliable vehicle speed reference.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Inter-wheel speed correlation | Sensor signals consistent within 5-10% variance | All four sensors show incoherent or contradictory speeds |
| Sensor signal quality | Clean, stable waveform with proper amplitude | Absent, severely degraded, or noise-filled signals from all sensors |
Code C1091 is classified as a serious fault. If your check engine light is flashing — not just steady — pull over safely and do not continue driving. A flashing CEL indicates an active misfire or critical failure that can cause catalytic converter damage within minutes or permanent engine harm within miles. Contact a certified mechanic immediately. Do not attempt roadside repairs on high-severity codes unless you are trained to do so.
Once the fault is repaired, C1091 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.