Why is my car's heater blowing cold air when the heat is on?

There are several possible reasons why your car's heater may be blowing cold air when the heat is on. Some common causes include low coolant levels, a malfunctioning thermostat, a clogged heater core, or a problem with the blend door. It is recommended to have a professional mechanic inspect and diagnose the issue to determine the exact cause and necessary repairs.