How to Determine if a Coolant Temperature Sensor is Faulty?
2 Answers
When the following symptoms occur, the coolant temperature sensor is likely faulty: 1. Difficulty starting the car. When the coolant temperature sensor fails, it often displays the temperature signal of a hot engine during a cold start. The ECU cannot receive the signal to provide a richer air-fuel mixture and can only supply the engine with a leaner mixture suitable for a hot engine, making it difficult to start the engine when cold. 2. Short circuit in the coolant temperature sensor or wiring causes the temperature gauge needle to point directly to the highest position; if the coolant temperature sensor or wiring is short-circuited, the temperature gauge needle does not move at all; if the coolant temperature sensor is covered with contaminants, the temperature reading will be inaccurate. 3. Affects the EGR valve. The function of the coolant temperature sensor is to convert the coolant temperature into an electrical signal, which is input to the ECU to: adjust the fuel injection quantity; increase the fuel injection quantity at low temperatures; adjust the ignition timing advance; increase the ignition timing advance at low temperatures and retard it at high temperatures to prevent knocking; affect the idle control valve; at low temperatures, the ECU controls the idle control valve based on the signal from the coolant temperature sensor to increase the idle speed.
I recommend first observing the coolant temperature gauge reading and engine behavior. If the temperature indicator consistently shows too low or too high, or if the fan operates unnecessarily during cold starts, it may indicate a faulty sensor. Next, use a multimeter to test the sensor's resistance: when the engine is cold, the resistance across the sensor terminals should be relatively high, such as several thousand ohms; as the temperature rises, for example, to the normal operating range around 90 degrees, the resistance should drop to a few hundred ohms. If the resistance remains unchanged or the readings are abnormal, it indicates a sensor failure. Additionally, check the sensor connector for corrosion or looseness—if found, clean or replace it immediately. Finally, use an OBD-II scanner to read the trouble codes. If codes P0115-P0118 appear, it confirms the fault. Address this issue early to prevent engine overheating damage—it saves money and ensures safety. Don’t wait for the coolant temperature warning light to take action.
