Hi cobbsski,

If my description of what your heater is doing please read through to the end.

The heater is turned on and a room temperature is selected.
You can hear the noise of a small fan start, which shortly after the heater starts.
After a short period of time the heater stops and the red led begins to flash and shuts itself down.

This type of heater has a balanced flue, which means it draws air in one pipe and exhausts it out another.
Have the balanced flue checked for blockages, the problem will most likely be in the flue pipe which draws the outside cool air into the heater and not the flue pipe which blows out the hot air.
it may be full of cob webs or something else which restricts the air flow to the heater.
A service person can perform a quick test to prove the above theory. At the rear of the heater there are two pipes, one is an insulated metal pipe and the other is plastic pipe.
Disconnect the plastic pipe from the flue box and restart the heater.
If the heater runs normally this will confirm that there is a blockage.