I have a Coleman 7680 gas furnace that will only run for about 30 seconds at a time. When we first turned it on this year we noticed that the air blowing out was not very warm. Then noticed the cycles. Tried replacing the thermostat but it still only runs for maybe 30 seconds at a time. We live in a mobile home that is pretty old but it worked fine last year and cannot figure this out. Pilot light lights and when it kicks on the flames work but it just won't heat the home. Any suggestions?

Maybe The Limit Switch Is Bad.. cutting Out The Burner Too Soon.

Try setting the cycle setting inside your thermostat to a higher setting so the cycles last longer. Maybe the reason why your house is not getting warm is because the furnace quits before it gets a chance to get going

You can quickly sort out whether it is the furnace or thermostat by jumpering the R and W terminal at the furnace. If the old furnace only has 2 unlabeled wires, jumper them. The furnace should come on and stay on as long as the terminals are connected. Does the blower come on? Older furnaces had 2 limit switches. One turned the blower on after the gas came on and heated up the furnace. The other shut the gas off if the blower didn't come on and the furnace overheated. Check them.

Dirty or blocked A/C coil. Mobile homes are famous for this problem due to the lack of or poor air filtering ability. This is a downflow furnace and the A/C coil is mounted under the furnace. When dirty the heat will not blow past the coil causing a shut off the furnace on high limit or the restriction has cause the limit to become weak and trip to soon. Just my 2 cents worth.