I am actually writing for my sister with no internet. I do not have the model of the furnace. They came to their cabin today to find it frozen. The diagnostic light on the Nordyne Intertherm Furnace will flash twice when the furnace is turned on. The furnace will run for a few seconds and the shut off again. Could not find a manual anywhere with diagnostics in it on the internet either. Any help would be appreciated. Thank you.