Heater blows cold?

Heater only blows cold or warm air, never hot. Can anyone tell me why.:uhoh:

Possible causes

low coolant level
clogged, resticted heater core
worn water pump
thermostat stuck open
malfunctioning blend door

I have has this problem twice on a '97 sable. The problem was a rust/corrosion build up that backs into the heater core. The first time, 4 years ago, the dealer "acid flushed" it for $500. Two months ago I took it to Rad-Air and they did it for $200. I would recommend a radiator flush regularly - which I didn't do. The car just seems to be prone to the buildup. Good Luck.