Coleman gas funace, model #7680C856 for mobile home worked great last year but since cold season hit, can't keep home warm (especially at night where it has been as low as 50 degrees). Thought it might be the thermostat so replaced that with new one but same thing. Thermostat clicks, burners light, blower doesn't activate for about five minutes then runs about 30-60 seconds and shuts off. Heater will kick back on about 15 min. later but same thing over and over. Tried shorting upper and lower limit switches - one shut the gas off when shorted - the other lit the burner. Please, please help if you can. Currently using one itty bitty electric space heater to heat me and kids. Thanks if you have any suggestions!!

Are you sure that is the model number not the serial number. What kind of ignition does it have, standing pilot, hot surface ignition. Does it have a PC board?

On the fan housing is a switch with a lever which hangs down into the housing. This is called a sail switch. When the thermostat turns on the fan starts and the air flow will trip this switch on which inturn will turn the heat supply on. These offen become dirty. Check with a ohmn meter and clean or replace

If the thermostat has a fan auto/on switch, try switching it to on and see if the furnace will stay on. Keep you warm until you get back with the info Northern Heat asked for.

I live in a trailer, have a Coleman, not sure ofmodel number. Everything to heat and blow like it should, however when furnace shuts off after reaching temperature, it gets cold and feels outdoor air is coming from hot air vents. No cold air returns. Is something suppose to close to prevent outdoor air from coming in when no longer needed to warm trailer? Joy

Ductwork under trailer and/or insulation could be damaged causing the cold air problem. Yes, there is no cold air return, it is actually the top door of the furnace where the filter goes.

Sail switches are very rarely used in coleman furnaces. It is also unclear if this is a mobile home furnace or residential home, they have made both.