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How to Determine if a Coolant Temperature Sensor is Faulty?

2 Answers

AliyahMarie

07/28/25 10:50pm

The function of the coolant temperature sensor is to convert the coolant temperature into an electrical signal, which is then output to the electronic control unit to adjust the fuel injection quantity, modify the ignition advance angle, and influence the idle control valve, among other functions. If the coolant temperature sensor malfunctions, the following symptoms may occur: 1. The temperature gauge needle does not move or points to the highest position. 2. Difficulty in starting the engine when cold. Due to a faulty coolant temperature sensor, the signal displayed during a cold start still indicates the temperature of a warm engine. As a result, the ECU cannot receive the signal for a rich air-fuel mixture and can only provide the engine with a leaner mixture. 3. Increased fuel consumption, unstable idle; difficulty in accelerating, even when the throttle is fully depressed, the engine RPM does not increase.

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DelLaura

08/12/25 3:17am

That day when I was fixing my old buddy's Corolla, I encountered a situation where the temperature gauge went on strike. The most obvious signs were the temperature needle on the dashboard either staying lifelessly still or suddenly going crazy and shooting up to the red zone. I also noticed the fan acting strangely: on a cold day, right after starting the car, the cooling fan would start buzzing and spinning wildly, or after driving for half an hour, the fan wouldn't turn on at all. These abnormal phenomena are almost certainly caused by a faulty coolant temperature sensor. When repairing cars, I usually check if the warning light is on first, then use a multimeter to measure the resistance changes of the sensor in both cold and hot states. Normally, as the temperature rises, the resistance should steadily decrease. If the resistance doesn't change at all or fluctuates wildly, it's a clear sign that it needs to be replaced. By the way, rusty or loose connectors can also cause similar issues, so remember to check for contact oxidation when unplugging the connector.

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