My heater was working fine this past winter. And I even turned it on in October once and it was working fine. I live in a mobile home and have an Intertherm heater. It keeps telling me the pressure switch is open and won't turn on. I don't even know what the pressure switch is! It does the normal "humming" sound when first turned on. Just heater or blower never comes on after that. Any help would be greatly appreciated! Thanks in advance. A/C works fine. Not sure if that matters.

You have a small blower that draws in combustion air. If it doesn't close a pressure switch, the gas won't come on. The humming is other the small blower or its motor trying to start. Track the blower down. It should be between the burner and where the vet goes out of the furnace. The pressure switch is between it and the burner, often at the end of a hose.