APU Heater

Sounds like the heater itself. Typically they have two stages of heat. Just guessing but it may be hitting only the low stage and then its trying to go into the second but not hitting anything. They're easily replaced and if memory serves correct, but that expensive. Most trucks with shore power will only use the first stage, just designed to keep things from freezing. Maybe 45° when it's 10° outside.

 

What APU do you have ?

 

Do you have an Espar bunk heater? It will be a small black box under the bunk with a duct in and duct out….

 

Espar requires a little bit of maintenance to keep going. They usually get ignored until not working properly.

 

The larger Espars typically do not seem to require maintenance in my experience. I know the smaller ones constantly clog the little screens. One trick is to prime it with kerosene which will burn hotter and burn off the gunk but the heat has a tendency to weld the screen to the casing and then replacing it, which you'll eventually need to do, is very difficult.
Then there's the $100 Chinese heater option, but I've heard horrible things. There is a unit called the Parking Heater Pro (PHP). They are made in China but to strict specifications and they are about half the price of an Espar. I have the large going into 3 winters now.

 

running clear unscented K-1 kerosene through your espar for about 45 minutes is the accepted way to get carbon deposits burned out, do it once a year. I've never had the screen clog up, I usually replace the screen and glow plug every 3-4 year. Remember to run it once a month even during the heat of summer to keep it in good working order. Doesn't take much to keep them going.

 

I had one of the smaller Espar brands once and it used to clog up and error about every other month. I had another 4kw Espar, the big one for 4 to 5 yrs and never had to do anything to it. And, yes, fire it a couple times in the off season.