I have an older model Night & Day model c100u series 510 gas furnace. The burner won't stay lit when the thermostat kicks on. I replaced the thermocouple but the burner still lights then immediately goes out. Any help?

Which burner - main or pilot? If main, try this: Locate the thermostat control wire connections at the furnace, remove wires from "R" and "W", and place a jumper across the terminals. This will bypass the thermostat circuit. Does the unit run continuously now? If so, you have a problem with the thermostat and/or wiring. Post back results.

Upon further investigation it became clear that my blower motor was fried, so apparently the main burner shut down when the fan would not come on. I have a new motor on order, when I get it installed we will see if that does the trick. Thanks for your help.