I have a 2003 Elantra 5 spd. The heater has been acting weird since last winter. I'm trying to sell the car now and it's hard to do when the heater doesn't work right... So while sitting idle with the heater set on 1 or 2 it blows hot air. But when you turn it to 3 or 4 it starts getting colder until it's not warm anymore. Then you drive the car and it will blow hot air on the the higher settings. Anyone got any ideas?

 

Clogged heater core.
Need to do a heater core flush