A car heater works by blowing air from the heated engine over a heater core and funnelling it into the vehicle compartment. All components need to be functioning properly for the air to heat the car interior fully. Usually this will take a few minutes. If the heater does not respond accordingly, there are a number of steps you can take to troubleshoot this problem before an expensive trip to the mechanic.
Check to see you have enough coolant in the radiator. If not, add the proper amount.
Start the car and turn the fan and heater to the highest setting. Listen for the fan to engage when you flip the heater switch. If you do not hear it, it could mean that a fuse has blown.
Check the fuses, usually located in a fuse box beneath the steering column. Pull out the fuse connected to the heating system and check if it is broken. Replace as necessary.
Place your hand above the black or red hoses connected to the heater core at the firewall. The hoses should both be hot. If they are not, the coolant is not circulating correctly and it is time to have a professional replace the heater core.
Check the inside floor of the vehicle for wetness or a strong odour. If these signs are noticeable, you may need to replace the heater core.
Check the diagram on the fuse box to see where the fuse for the heater is located. Make sure to replace a fuse with one of the same amperage. Front-wheel drive or rear-engine configuration are more difficult to diagnose, and you might have to take your vehicle to a mechanic. If there is no heat blowing into the passenger compartment, it could be because the thermostat is stuck in the open position and might need to be replaced. If the heater and defroster are not working, it could be that the gate on the heater duct is malfunctioning. A silent fan could also be due to a malfunctioning motor an electrical problem. Sometimes replacing a fuse might not fix the problem. The circuit may have overloaded. Usually, if the new fuse blows right away, there could be a short in the system. Other potential car heater problems can include dented or damaged heater hoses, and a malfunctioning water pump that is too weak to fully circulate the coolant.
Never place a temporary barrier, like cardboard, in front of the radiator to try and fix the problem. This can cause overheating.