When the 2023 Mercedes-Benz GLC sets P0118, the ECT sensor is sending higher voltage than expected. This indicates low resistance, interpreted as extremely high coolant temperature. The ECU may activate cooling fans unnecessarily and lean out the fuel mixture.
Understanding P0118
P0118 indicates ECT circuit voltage exceeds normal thresholds. The ECT thermistor has low resistance when hot. If voltage is abnormally high (low resistance), the ECU thinks coolant is extremely hot, possibly triggering limp mode or fan operation even when cold.
Mercedes GLC Cooling System
The 2023 GLC's turbocharged engines require precise temperature management for optimal performance and emissions. The sophisticated engine management system uses ECT data for numerous calculations. False high readings affect many systems.
Common Causes of P0118
A shorted sensor or circuit causes low resistance, resulting in high voltage. If the sensor element fails shorted or the signal wire grounds, readings indicate extreme heat regardless of actual temperature.
A failed ECT sensor with internal short outputs high voltage continuously. The thermistor element can fail in a shorted state from age or thermal stress.
Water intrusion in the connector can create a short between terminals. Inspect for moisture, which is common given the sensor's location near coolant.
Diagnostic Approach
Monitor ECT readings on a cold engine—if showing extremely high temperatures (250°F+) immediately after starting, suspect a short. Disconnect the ECT sensor to see if readings change to extreme cold (open circuit). If so, the sensor is shorted. Check wiring for shorts to ground.