It's called a heater core-but there's nothing "little" about the job of replacing one.

As we continue our Cooling System Revival, we're getting right into the core of things: the heater core.

What exactly is a heater core? What does it look like? Where is it hiding? How does it work? And why can something that looks like a tiny radiator turn into a major repair bill?

The heater core is part of your vehicle's cooling system, using hot engine coolant to provide heat inside the passenger compartment. But when it becomes restricted with rust, sludge or deposits, you may lose heat - or lose coolant altogether.

And here's the ugly part: getting to the heater core.

On many vehicles' the heater box is buried deep behind th ...