Not much to go on but...
First, BE SURE YOUR VEHICLE'S COOLING SYSTEM IS FULL!
If not, FILL IT! If it keeps 'disappearing' after a few days or a few weeks, your engines gaskets are probably worn beyond ignoring. (I killed my Malibu by ignoring the issue, I loved my Malibu... sigh...). They MUST be replaced before the engine oil and coolant 'become one with each other' (BTW... this is bad).
If your engine is full, and your coolant hoses become firm (firm by means of pressure from heated gases and liquids expanding in your cooling system) after running a while...
I would start with the Thermostat...
If your engine is not running anywhere near 195 degrees Fahrenheit after running for about 15 minutes, your thermostat is most likely stuck open and , therefor, your engine is not warming up to operating temperature. Just watch your temperature guage on your next trip to the market. My 2003, Chevy, 2300 V6 engine needs to warm to 150-160 to even start to heat the vehicles interior and during very cold conditions, it may take up to 175-180 before good 'hot' air gets going.
Be aware other issues can cause this as well. Water pump failures and blockages in the system can cause COLD air blowing from the heater as well. If the air is cool or WARM, go with the thermostat first. It's inexpensive, and 'relatively' easy to do.
Good Luck!
With my car, it was Heater matrix clogged with radiator sealant. The workshop cleaned it and it's start working fine!
Get the air out of your heater core.
the heater blows warm air when fan is on low but blows cold air when fan is turn to higher setting
heater hoses might have been disconected
The most common reason a automobile heater blows cold air is the water level is low. Check the water level and fill to the appropriate level.
we changed radaitor & thermastate , now heater blows cold air
ihave a 1986 chevette that when i turn defrost on it blows out heat and when i put heater on it blows out only cold air? ihave a 1986 chevette that when i turn defrost on it blows out heat and when i put heater on it blows out only cold air?
you must have low coolant or a bad heater core
The heater may not be on - but the fan still blows air through the vents when they're open !
Whats the problem? Blows cold air with the heat settings on?
The most common reason the heater is blows cold air on a 1999 Ford Winstar is a bad thermostat or blocked heater core.
possibly thermostat stuck in the open position?
it has a dead raccoon stuck in it