what is the most likely cause for a car's heater to be blowing cold air?

Thermostat stuck.

That's an odd one. Check the water pump and the belts but I wouldn't bet that's the issue. The heater in the car is simply a small radiator. Hot water from the engine runs through it and a fan blows air over it and then into the car. There is also a duct to the outside world. To change the temperature, all you are doing is selecting how much air is directed through the heater core. I think that linkage is mechanical in that Chev but I'm not sure. There is also an option to recirculate the air through the car rather than taking outside air. Try that and see what happens. If you still get warm air then there is something wrong with the duct work or similar. If it's still cold when the car stops then you will have to look elsewhere. That's the best I can do without seeing the car but I'd like to know the problem and solution if you can remember to e-mail me. Thanks and good luck.

The way a heater works in water cooled car is simple. It is just a small radiator. When you turn on the heater you are adjusting two things. The speed of the fan blowing over this small radiator called the “heater core.” When you turn on the heater you are opening a valve which lets hot water from your cooling system into your heater then the fan blows cold air over the heater core and you have heated the inside of your car.

Some possible causes

Insufficient coolant

Bad thermostat

Leaky heater core

Clogged heater core

Heater control valve not allowing hot water through heater

First make sure that you have enough coolant in the radiator. Start the engine and after it warms up turn on your heater. Feel the heater hoses to see if they are getting hot.

Make sure that you cooling system is working properly and that your car isn’t over heating.

The heater hoses are the small hoses that run from passenger side firewall to your engine and water pump. After your car is warm feel them to see if they are getting hot. Most likely they aren’t and that could be a sign of either a clogged heater core or a bad heater control valve assuming that you have enough coolant in your radiator. Even if you have a faulty thermostat you should feel some heat coming through.

I’m trying to give you some tips but because it could be many different causes it is really hard to trouble shoot here. These are just some suggestions of things to try to narrow down what your real problem is. Another suggestion is bring it to a mechanic and see if you can get a free estimate. If so have them list or tell you what needs to be replaced. If you have a leaky heater core and don’t have the money to fix it you can always bypass you heater and fix it later when you have the money. I doubt you have a leak or you would have mentioned it earlier. If you do have the money I would suggest that you bring your car to a respected mechanic to fix this. And don’t forget it never hurts to get several estimates.

On newer cars, hot water flows through the heater core all the time. The amount of heat you receive is regulated with air doors in the heater/ac system. The doors may be stuck, or no vacuum to open them is present.

check coolant level if full depending on what type car heater control valve is bad or lost vacuum or your heater core is plugged up

check your anti-freeze level in your rad. the coolant circulates through the heater core and if there is not enough coolant it will not work efficiently. and will not blow warm air.

Heater coil is broken.

lack of coolant in the radiator or an air lock.

your thermostat has gone bad. 5 dollar part. can be fixed in 1/2 an hour...

Radiator not full