PRNDL Input #4 Circuit Failure
The transmission control module can't read the gear selector position sensor signal properly, like a broken switch that won't tell the car which gear you've selected. This prevents the transmission from knowing what gear to engage.
The TCM monitors the PRNDL (Park, Reverse, Neutral, Drive, Low) sensor input #4 circuit for valid voltage signals corresponding to each gear position. The ECU compares the signal voltage to expected thresholds and detects circuit opens, shorts, or invalid voltage patterns that indicate a wiring or sensor failure.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| PRNDL Input #4 Voltage | 0.5V to 4.5V (varies by position) | Below 0.1V, above 4.9V, or no signal detected |
| Signal Continuity | Continuous valid signal | Open circuit, intermittent loss, or shorted to ground/power |
Code C1861 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, C1861 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.