The heater in your car keeps you warm in the winter when it's cold. The heater system uses coolant from the engine's cooling system to heat the passenger cabin. It does this by routing some of the coolant from the engine cooling channels into a heater core, which acts like a miniature radiator. Then, a fan blows on the heater core and blows the warm air into the cabin. In order for the system to work, a heater core valve must be working normally. If the valve breaks, then your system won't work at all. Before replacing any parts, however, you should troubleshoot the heating system.
Start your car's engine and let it warm up. When the needle on the temperature gauge sits at or near the middle of the gauge, then the car is sufficiently warm.
Turn the heater to the hottest setting.
Turn the fan speed to high.
Check to make sure that there is heat coming from the system. If there is no heat coming from the system, you may have blown a fuse.
Open the fuse panel under the steering column and locate the fuse for the heater and fan system. Use the fuse diagram to locate the fuse for the heater.
Pull the fuse for the heater using the fuse pullers in the fuse box.
Check the metal strip inside the fuse. If it is broken or burnt, replace the fuse with a new fuse of the same amperage. If the heater still doesn't work, you may be having trouble with the coolant making its way into the heater core.
Open the bonnet and place your hand over the top of the heater hose running from the radiator to the firewall. Do not touch the heater hose because, if there is coolant in it, it will be very hot. Instead, just put your hand over it close enough to feel if there is any heat coming off the hose. If there is, then there is a problem with the heater core valve, which will need to be replaced.
Get into the passenger side of the car and feel the floorboards. If they feel wet, if you smell a sweet smell or you see green liquid, then the heater core is leaking and needs to be replaced.