Engine Coolant Temperature Signal Missing/Fault
Your engine's coolant temperature sensor isn't sending a signal to the computer, like a broken thermometer that can't report the temperature. Without this data, the engine can't adjust fuel mixture and timing properly.
The ECM monitors voltage from the coolant temperature sensor (typically a thermistor) to determine engine temp. It expects a signal within a specific voltage range that correlates to -40°F to 250°F. If no signal or out-of-range voltage is detected for a set duration, the fault is logged.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Sensor Voltage | 0.2V to 4.8V (correlating to engine temp) | No signal, <0.1V, or >4.9V for >2 seconds |
| Signal Continuity | Continuous, stable signal stream | Intermittent or missing data packets |
Code C1781 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, C1781 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.