The first step is to get you and the car to a, preferably, heated garage. Drive the car long enough to warm up the engine and heater. Setting the heat on high and setting the blower at medium or higher are good ideas to check its heat output.
Monitor the heat output by inserting thermometers into both the driver and passenger side vents. While it is possible to have a higher reading, somewhere between 115 degrees and 120 degrees is usually an acceptable temperature. If your car’s heating system is registering in this area, great, if not read on.
Excellent resources to help quickly and accurately pinpoint the problem and optimize heat are the owner manual and manufacturer’s website. The answer may not be there and in that case it’s time to check out what’s going on under the hood.
Looking blankly at the heater probably won’t do any good, so the first place to look is at the coolant levels. Be sure the coolant level in the reservoir is up to manufacturer’s specifications. Even if this isn’t the heating problem, proper coolant levels will help the heater run warmer.
Also be sure the coolant is clean. If it has been replaced regularly you should be in good shape. Another issue with coolant is to closely monitor its temperature while you are at it.
If the car’s heating system is still not warm enough, make sure the temperature door motor is working correctly. On many models you can access this area by either lowering the glove compartment or looking under the dashboard by removing the panel. Here is where the owner’s manual would come in handy.
While you are checking out the temperature door motor keep an eye on the shaft operation as well. If you can’t see or feel the motor running or the shaft operate then you have a good idea that this is your problem. Most of the time correcting this situation is simple by replacing the assembly. Typically it isn’t necessary to replace the motor too.
Other important features of the heating system to check are the hoses. You will want to be sure there is no debris clogging the system. Simply detach the hoses and put a garden hose through them to knock any build up loose. If after cleaning the hoses the heat output is still to low, then the heater core probably needs to be replaced.
If all else fails, calling a professional mechanic is your best option. No one wants to be Popsicle all winter long.