why does the heater blow cold air

first,check the coolant level,must be full,check it when cold....if it's full,suspect failed thermostat.

Hello, you could have an engine performance problem, such as a thermostat not functioning properly and not allowing the engine to get to the proper operating temperature, resulting in cold air. Or, you could have a ventilation system issue, where the doors in the dash that meter air between hot and cold are not working properly, leaving it to the cold side. Either way, an inspection / diagnostic test needs to be performed to find out what is going on.