Coolant Temperature Sensor

📖 YOUCANIC Automotive Glossary

Coolant Temperature Sensor (CTS)

The coolant temperature sensor is a thermistor mounted in the engine block or cylinder head that measures the temperature of the engine coolant and sends a continuous voltage signal to the ECU. The ECU uses this data for a wide range of functions — enriching the fuel mixture during cold starts, controlling cooling fan activation, adjusting ignition timing based on temperature, and triggering the temperature gauge on the dashboard. A failing CTS can cause the ECU to think the engine is always cold, leading to a permanently rich fuel mixture, black smoke, poor fuel economy, and fouled spark plugs. Conversely, if it reads falsely hot, the ECU may lean out the mixture causing misfires and potential engine damage. Other symptoms include an erratic or pegged temperature gauge, cooling fans running constantly or not at all, hard cold starts, and a check engine light with codes like P0115, P0116, or P0117.