You must look for symptoms of a bad thermostat in the car if your car heater blows cold air. You need to check if: If your car engine is overheating in no time, or If the temperatures are changing dramatically, or The coolant is leaking around the thermostat, or Car heater not blowing hot air. If any of these symptoms are visible, take your car to the mechanic and get it checked. Of course, you can also check it at home, but if you are not very handy with car parts, professional help is recommended.