Coolant Temp Out of Range
Your engine's coolant temperature sensor is reading a value outside the expected operating range, like a thermometer giving an impossible reading. The ECU can't trust the coolant temp data and triggers this warning to prevent engine damage.
The ECU monitors the coolant temperature sensor (CTS) signal voltage to verify engine warmth state. The sensor resistance changes with temperature, generating a voltage signal that the ECM compares against calibrated thresholds. If the signal falls outside acceptable limits (too high or too low), a fault is logged.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Coolant Temp Sensor Voltage | 0.5–4.5V (equivalent to -40°C to 125°C) | Below 0.1V or above 4.8V |
| Coolant Temp Rate of Change | Gradual rise over 5–10 minutes | Sudden spikes or drops within seconds |
Code C1774 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, C1774 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.