Engine Coolant Temperature (ECT) Sensor Circuit Intermittent High Voltage/IAT - B Circuit High Input
Your engine's coolant temperature sensor is sending an unusually high voltage signal to the computer, like a thermometer stuck at an impossibly high reading. This intermittent fault means the problem comes and goes, making it harder to diagnose.
The ECM monitors ECT sensor voltage to determine engine temperature and adjust fuel mixture accordingly. The sensor normally produces lower voltage as temperature rises; high voltage indicates a cold engine. The ECU detects when voltage stays abnormally high or spikes intermittently, signaling a circuit fault.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| ECT Sensor Voltage | 0.2–4.8V (varies with temperature) | >4.8V or intermittent spikes above threshold |
| Coolant Temperature | −40°C to 125°C | Implausible voltage-to-temperature correlation |
Code P1120 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1120 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.