Why is the heater blowing cold air on a 2001 Ford F-150?

Answer 1

Either the heater core is not getting hot, or the blend door is not diverting air through the heater core.

The Heater core is difficult to replace on the F150 since it is buried in the plenum box. Getting to the core involves removing the steering wheel, steering column, complete dash panel, evacuating the AC, draining the radiator, and finally removing the heater box. Once you have the box out(8-12hrs), the core is simple to replace.

You want to be sure that the core is the problem before you undertake this process. If there is a strong smell of antifreeze in the cab or a puddle of coolant on the floor, the diagnosis is easy.

Check that the radiator is full, not the overflow tank, the radiator. On a cold engine you can remove the radiator cap and check the level. It should be full. If there is air in the system it will affect cab heating and engine cooling.

Another good check is to feel the two heater hoses going into the firewall from a cold start. The hoses should warm up at about the same rate and get uncomfortably hot-close to radiator temperature.

If the core appears to be OK, the next concern is with the blend door that diverts air through the heater core. This is a common failure on the F150.