P1117 – Intermittent Engine Coolant Temperature Sensor

Repair Procedure:

1. Preparation and Safety:

   • Cold engine (risk of burns and pressure in the cooling circuit).

   • Basic tools + digital multimeter (DMM).

   • Vehicle-specific technical documentation (electrical diagrams, ECT sensor resistance/voltage values).

2. Visual Inspection (Priority):

   • Coolant Level: Check and top up if necessary. A low level can expose the ECT sensor and distort its reading.

   • Physical inspection of cables and connectors:

     • Disconnect the ECT sensor connector and the PCM connector (if accessible).

     • Look for corrosion, bent/broken/retracted pins, frayed or melted wires, damaged insulation. Clean corrosion with an electrical contact cleaner.

     • Check the tightness of the connector to the sensor (damaged O-ring?).

   • ECT Sensor Inspection: Coolant leak at the threads or seal? Physical damage?

3. ECT Sensor Test (Resistance – Cold & Hot Engine):

   • Disconnect the sensor’s electrical connector.

   • Measure the resistance between the sensor terminals with the multimeter in Ohms (Ω).

   • Cold Engine: High resistance (e.g., several kΩ at 20°C – check specific values in the technical documentation).

   • Engine at Operating Temperature: Much lower resistance (e.g., a few hundred Ω at 90°C – check the documentation).

   • Compare your measurements to the manufacturer’s specific values. Infinite resistance (open circuit) or zero (short circuit) indicates a faulty sensor. A value outside specifications also indicates a defective sensor.

4. Electrical Circuit Test:

   • Power Supply (VRef – often 5V):

     • Turn ignition ON without starting.

     • Reconnect the connector to the sensor or use a backprobe pin.

     • Measure the voltage between the VRef wire (usually a light-colored wire, check diagram) and ground. Expected: ~5V. If 0V: short to ground or open circuit between PCM and sensor, or faulty PCM. If 12V: short to battery voltage.

   • Ground:

     • Ignition ON.

     • Measure the voltage between the sensor’s ground wire (usually a black or dark wire) and the chassis ground. Expected: ~0V. If high voltage: poor ground circuit (corrosion, loose connection).

   • Signal:

     • Ignition ON.

     • Measure the voltage between the sensor’s signal wire and ground. Cold engine: High voltage (e.g., 3-4.5V). Hot engine: Low voltage (e.g., 0.5-1.5V). Compare to specifications.

     • A constant voltage (e.g., 5V, 0V, or VRef) or a value outside specifications indicates a circuit or sensor problem.

   • Continuity/Isolation Test:

     • Ignition OFF.

     • Check continuity (≈0Ω) for each wire between the ECT connector and the PCM connector (VRef, Signal, Ground).

     • Check for the absence of short circuits (∞Ω) between each wire and ground or +12V.

5. Thermostat Check:

   • A thermostat stuck open prevents the engine from reaching its normal temperature, potentially generating a code related to a persistent “abnormally low” ECT reading.

   • Monitor the engine temperature rise on the dashboard or with a diagnostic tool. A very slow rise or a maximum temperature that is too low (<80°C while driving) is suspicious.

   • A physical test of the thermostat (removing it and immersing it in boiling water to see if it opens) is more conclusive.

6. PCM Diagnosis (Last Resort):

   • PCMs rarely fail. Consider this cause only if:

     • All other checks are OK (circuits, power supplies, ground, sensor).

     • The fault persists after replacing the sensor.

     • Thorough circuit tests (with a precise diagram) point to the PCM.

   • Before condemning the PCM, check the quality of the connections and the PCM’s own ground.

Summary of Causes & Corrective Actions:

After Repair:

1. Clear the fault code with a diagnostic tool.

2. Perform a drive cycle to verify that the fault does not reappear and that the engine temperature behaves normally.

3. Check that the Malfunction Indicator Lamp (MIL) no longer illuminates.

This systematic approach, starting with the simplest and most common checks, allows for effective diagnosis and repair of the P1117 code.