P0117 Engine Coolant Temperature Sensor Circuit Low Input

What Does Code P0117 Mean?

Diagnostic Trouble Code P0117 indicates that the Engine Control Module (ECM/PCM) has detected an abnormally low voltage signal from the Engine Coolant Temperature (ECT) sensor circuit. Because the ECT sensor is a thermistor, a very low voltage is interpreted as an extremely high coolant temperature. This may be caused by a short to ground, a faulty sensor, or damaged wiring.

Symptoms

• Check Engine Light (MIL) illuminated
• Cooling fan may run continuously
• Hard starting
• Poor engine performance
• Reduced fuel economy
• Engine may enter fail-safe mode
• Incorrect coolant temperature reading on the scan tool

Possible Causes

• Faulty Engine Coolant Temperature (ECT) sensor
• Short to ground in the signal wire
• Damaged or melted wiring harness
• Corroded or contaminated connector
• ECM internal fault (rare)

Typical Circuit

ECM (5V Reference)
        │
        ├────────────► ECT Sensor
        │
Ground ─┘
        │
Signal ─────────────► ECM

Signal Flow

Coolant Temperature
        │
        ▼
ECT Sensor
        │
Voltage Signal
        ▼
ECM
        │
Fuel Injection
Ignition Timing
Cooling Fan Control

How to Diagnose

1. Visual Inspection

• Inspect the ECT connector for corrosion or coolant contamination.
• Check wiring for contact with the engine or exhaust components.
• Inspect for damaged insulation causing a short to ground.

2. Live Data Test (Autel)

• Open Diagnosis → Engine → Live Data.
• Select Engine Coolant Temperature (ECT).
• If the engine is cold but the scan tool displays approximately 140°C (284°F) or higher, the ECM is receiving a low-voltage signal.
• Disconnect the ECT sensor. If the displayed temperature changes to approximately -40°C, the sensor is likely faulty. If it remains very high, inspect the wiring for a short to ground.

3. Multimeter Test

• Verify the 5V reference voltage.
• Check sensor ground continuity.
• Measure sensor resistance and compare it with the manufacturer's specifications.
• Check continuity between the signal wire and ground for an unintended short.

4. Smoke Test (If Applicable)

A smoke test is not normally required for P0117 because this code is an electrical circuit fault. Perform a smoke test only if an intake leak is also suspected.

Common Repairs

• Replace the faulty ECT sensor.
• Repair shorted or damaged wiring.
• Clean or replace the electrical connector.
• Repair damaged harness insulation.
• Clear the DTC and verify proper coolant temperature readings during a road test.

Severity

Moderate to High. Incorrect coolant temperature information can cause excessive fuel consumption, poor engine performance, improper cooling fan operation, and may increase the risk of engine damage if an actual overheating condition is not detected.

Important Note

ECT sensor resistance values, connector pinouts, wiring colors, and voltage specifications vary by manufacturer. Always consult the factory service information for the specific vehicle before diagnosing or replacing components.