Was out spot lighting last night and all of the sudden my heater stopped working. After a little more driving I discovered this, it just isn't working at idle and doesn't kick in until around 1500 rpm. Anyone have any ideas?

 

Heater core is blocked, I would flush it real good. Possibly the whole coolant system if its been a while since you have done it. I had the same issue.

 

you are low of coolant, your heater core is clogged,needs back flushed into a pan, back flush out in the drive somewhere, your thermostat is stuck open,you have a bad water pump. Use that order