What can cause a car heater to not work?

Answer I will firs verify is the climate control is working (the dash board) controls, and then work my way up to more complicated issues. There are to important by pass that could be clogged also, if water has been used instead of coolant.
well for one it could be the heater blower motor or it could be the control under the dash that changes it from hot air to cold air if the spring spring breaks it will not get hot.
Hope that helps Try changing the thermostat. If it is stuck open or failed, engine temp will not rise. I had same problem on 95 explorer, changed thermostat myself and solved problem. Answer
It could be the water pump but most likely the antifreeze is low and needs to be filled up to be able to circulate through the heater core.
need more info. you could be low on coolant.for all we know. there are alot of different parts to a cooling system which bring heat to the cabin area. start simple. ck fluid level make sure it is full when cold. 2. when running does it circulate through the hoses. 3. are your heater hoses hot. both. 4. does your fan blow in all possitions & does air come out from your vents?

Answer
A heating system works by diverting hot coolant through a heating grille where it warms up air which the fans blow in whichever direction and whatever speed you specify.
Basically there are two parts to the heater system, there is the heater tap, located in the engine compartment. It controls how much (if any) hot coolant goes through the heating system in your car. Probable causes are a leak in one of the hoses, the heating grille itself (you will smell coolant in the car) of the radiator is not sufficiently topped up with enough coolant to circulate through the system. If not turn your heater on to full, turn your car on for a short while, turn it off and top up the coolant. Also bleed the air out of the system if necessary.
The second part is self explanatory, the fans blow the hot air around the car if the problem is not with the first part above, it would be best to have it taken to an auto electrician.
first thing you need to check is your fuses .It is actually the heater motor, sam thing happened to my 2000 Alero