Can a bad coolant temperature sensor cause a car not to start? Yes, a bad coolant temperature sensor (CTS) can cause a car not to start, although it is not the most common reason for starting issues. The coolant temperature sensor monitors the engine's temperature and sends this data to the engine control unit (ECU). The ECU uses this information to adjust the fuel-air mixture and ignition timing, which are crucial for a proper start. If the sensor fails, it may send incorrect temperature readings to the ECU, leading to improper adjustments of these parameters, which can prevent the engine from starting. When the CTS fails, it often causes the ECU to think the engine is either too hot or too cold. If the ECU believes the engine is cold, it might enrich the fuel mixture (like in a cold start), leading to a flooded engine that struggles to start. On the other hand, if the ECU assumes the engine is already warm, it could lean out the mixture, making it difficult for the engine to i