Truck Heater Blowing Cold Air?

Brent

January 17, 20140 found this helpful

This usually means the antifreeze is old, and needs to be changed. It still has some ability to hold heat, so you get heat on low, but when you switch to high the extraction of heat is faster than the fluid can absorb. Replacing the antifreeze solution completely should solve this problem. You might also want to look at your thermostat, since a poorly functioning thermostat can create the same problem, except it never allows the heat to build up as high as it once did.