My Car Heater is not providing much heat / not working. What's wrong?

It could be any of a number of problems. A heater that's working properly should blow air that's about 75 to 100 degrees F. hotter than the outside air. If it doesn't, any of the following might be at fault: 

A low coolant level (often due to a leak or weak radiator cap), but may also be the result of not getting the cooling system completely filled. If you've just recently changed the antifreeze, check the coolant level in the radiator to see if the radiator is full. An air pocket in the heater core or a heater hose may be interfering with the flow of coolant through the heater core. 

One way to tell if the coolant is circulating through the heater core is to feel both heater hoses. Both the inlet and outlet return hoses should feel hot when the engine is at normal operating temperature and the heater is on. 

Refilling some cooling systems can be tricky. Some front-wheel drive and rear-engine applications require special filling procedures to eliminate the air pockets that become trapped in the heater hoses and heater core. To help vent the trapped air, some vehicles have a "bleeder" valve (or more than one valve) on the thermostat housing and/or certain hose connections. Opening the valve(s) allows air to escape as the system is filled. The valve(s) should then be closed when coolant starts to dribble out the valve. On vehicles that lack these special bleeder valves, it may be necessary to temporarily loosen the heater outlet hose so air can bleed out as the system is filled. 

An open thermostat or one that's too cold for the application (most vehicles today require a 190 to 195 degree F. thermostat). One way to tell if the thermostat is stuck open is to start a cold engine and feel the upper radiator hose. You should feel no coolant moving inside the hose until the engine starts to get warm. After several minutes, you should feel a sudden surge of hot coolant when the thermostat opens.   

A defective heater control valve. On most vehicles built since 1970, vacuum operated heater control valves are normally open unless vacuum is applied. This allows coolant to circulate through the heater core even when the heater isn't being used. To test the control valve, apply vacuum with a hand pump. If the valve fails to close, replace it. 

 A plugged heater core. Accumulated crud in the cooling system may plug the core and block the flow of coolant. The only cure here is to replace the heater core. To prevent the problem from reoccurring, the cooling system should be flushed and refilled with a fresh 50/50 mixture of antifreeze and water. Distilled water is best since it contains no minerals). 

An inoperative airflow control or inlet door in the heater ducting or plenum. If the defrosters aren't working either, you've found the problem. Sometimes all that's needed to fix the problem is to reattach or repair a loose cable or vacuum hose. On vacuum-actuated systems, however, the vacuum motor or control switch may be defective and require replacement. 

A defective blower motor. If the blower motor doesn't work (no sound/no air), the motor may be defective. Or, there may be an electrical problem such as a blown fuse, defective power relay, heater switch or resistor, or loose wire. A blown fuse is a symptom not a cause. A fuse blows when a circuit overloads to protect the wiring and other components against damage. If the fuse is blown, therefore, find out why it blew before replacing it. Always use a replacement fuse with the same amp rating as the original. If a new fuse blows as soon as the blower is turned on, there's an electrical short in the heater circuit or motor that should be investigated. If the fuse lasts a while and then blows, the fan motor is probably running hot due to worn brushes and/or bushings and should be replaced.
The motor itself can be checked by using a pair of jumper wires. Connect one wire to ground and the other to a source of battery voltage. If the motor fails to spin, it should be replaced.

A pinched or kinked heater hose. In rare instances, you might even find mis-routed hoses if somebody worked on the cooling system recently. 

A weak water pump (one with badly eroded impeller blades), or one that doesn't turn fast enough because of a slipping drive belt. 

An electric cooling fan that remains on all the time, or a clutch fan that's locked up and overcools the radiator 

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(original article link:http://autos.yahoo.com/maintain/repairqa/fluids_heat_air_conditioning/ques020_3.html)