If the windshield in a vehicle fogs up when the engine starts, it is time to consider a heater core replacement. Another sure sign that a vehicle is in need of a heater core replacement is a wet passenger side front floor board. Coolant often leaks from a broken or bad heater core. A hint of warm antifreeze aroma as a vehicle is running or directly after it's shut off is another sign of a needed heater core replacement. Perhaps the most overlooked sign of a required heater core replacement is simply a heater that fails to give off heat when the blower motor is switched to high.

In vehicles that are liquid cooled, the hot coolant is passed through a small radiator-type component called a heater core. An electrical blower draws air through the heater core and blows the warm air into the vehicle's passenger compartment. Occasionally this unit will develop a problem in the form of a leak or a plugged coolant passage. When this happens it is time for a heater core replacement. Older heater core units were made of brass and more difficult to damage; newer units, however, are manufactured from aluminum and plastic, requiring great care to avoid damaging the device when installing it.

Vibration as well as continued cold to hot temperature cycling can cause the heater core to develop a leak. The most common area where a heater core will leak is at the input and output lines where the rubber heater hoses connect to the heater core. This leak can come as a large leak that soaks the carpet on the front floorboards or as a subtle leak that simply remains in the heater ducting and fogs the windshield upon engine start up. In either case, the first sign of a leaky heater core signals that it is time for a heater core replacement.

While heater core replacement is not a difficult task, it can often be tricky to place the heater core in its proper mounting location. It is for this reason that many shade-tree mechanics take their vehicles to a professional garage for heater core replacement. These professional mechanics typically have access to specialty tools that make the job of replacing a heater core much simpler. With knowledge of a particular manufacturer's method of installing the original heater core, the mechanic is able to understand the proper sequence of parts to remove to make installation of a replacement heater core a much easier task.