GM 55591401 Sensor Assembly, Engine Coolant Temperature

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The coolant temperature indicator system includes a warning light or temperature gauge on the dashboard and a coolant temperature sending unit on the engine. In some models, the Engine Coolant Temperature (ECT) sensor also functions as the coolant temperature sending unit. If there is an overheating indication, it is important to check the coolant level and ensure that all connectors in the wiring harness between the sending unit and the indicator light or gauge are securely connected. When the ignition switch is turned to START and the starter motor is running, the indicator light should come on, indicating that the bulb is functioning properly. If the light does not come on, it could be due to a burned-out bulb, a faulty ignition switch, or an open circuit. Once the engine starts, the indicator light should go out and remain off, unless the engine overheats. If the light does not go out, it could be due to a grounded wire between the sending unit and the light, a defective sending unit, or a faulty ignition switch. It is also important to check the coolant to ensure it is correctly mixed, as plain water or coolant with a high water content may have a low boiling point and fail to activate the sending unit.