Thermostat stuck open: car not warming up and higher fuel use

Drivers searching “car not warming up” or “thermostat stuck open” often notice a cold cabin and poor winter economy. The engine never reaches proper temperature, so the ECU runs rich.

Symptoms

• Temperature gauge stays low on motorways
• Weak heater output
• Higher fuel consumption in cold weather
• Fan rarely runs because the engine stays cool

Stuck closed is more urgent

A thermostat stuck closed causes rapid overheating. Stop driving if the gauge climbs into the red—that is a different emergency than a cold-running engine.

Book cooling-system diagnostics on CarFix and describe whether the car runs too cold or too hot.