Acceleration Position Sensor Conflict
The engine computer detected conflicting signals from your throttle position sensors, like two speedometers in your car disagreeing on how fast you're going. This mismatch prevents the engine from properly controlling fuel and ignition timing.
The ECM monitors primary and secondary throttle position sensor outputs to ensure they track together within acceptable limits. When the sensors disagree beyond a calibrated threshold, the ECU cannot reliably determine driver intent and triggers a fault. This cross-check prevents erratic fuel injection and spark timing.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor voltage agreement | Primary and secondary within 0.5V of each other | Deviation exceeds 0.5V or sensors are inverted |
| Rate of change correlation | Both sensors respond at same speed | One sensor lags or leads by >50ms |
Code C1855 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, C1855 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.