After the thermostat is switched on the furnace fires up but will not stay on and keeps recycling

If oil or gas fired? There is a flame sensor or rollout switch that will not let the furnace stay on unless it senses the heat from the fire. DO NOT BYPASS THIS SWITCH/SENSOR.

Could be thermostat or safety switch(es) in the furnace. Are you sure that the pilot is staying lit? Are you sure the main burner is lighting? Are you sure the forced draft blower is running, and the blower-detection "flapper" switch is working OK? Any of these safety devices will cause the gas supply valve to close if they are not "satisfied" in a few seconds after the thermostat calls for heat. They keep your RV safe!

IT SOUNDS LIKE THE FAN CYCLE RELAY OR BOARD IS NOT MAKING THE FAN COME ON WHEN IT SHOULD THE BURNER SHUTS OFF PROBABLY BECAUSE IT REACHES THE HIGH LIMIT.

if it is shutting off abruptly the limit is either wired wrong or set wrong. If the fan comes on and blows the flame out there is a hole in your heat exchanger which can result in fire or carbon monoxide poisoning. Have an experienced tech take a look at it to be safe.

http://customer.honeywell.com/techlit/pdf/69-0000s/69-0721b.pdf

it sounds like the low temp limit switch, the one that cools the furnace down with the burner off and the thermostat open, when it cools to a point the fan stops, the other switch in burner area turns burner off/on, hope this helps

Hi, sounds like your flame sensor needs to be cleaned. It's a metal rod in the flame furthest from the igniter. Remove it and use steel wool or fine sand paper to clean.

Hmm...well need more info...is the furnace switch on???? The breaker may have tripped...try the fan on Manual at the thermostat and verify the power is on....it really can be many things...need more info

have the thermocoupler checked

Not much info to go on, but lets look at a few possibilities. Your thermostat could be turned to the ?Fan On? setting, make sure you put it in the ?Auto? mode. Or it could be bad, or there could be a short in the wiring. Open up the access panel on your furnace. Lets eliminate the thermostat from the equation by disconnecting the red, white and if you have a yellow wire at the furnace. Mark down which wire went where. The furnace will have a door safety switch that will have to be taped temporarily closed to test the furnace. With a short piece of wire (2-3" long), strip off 1/2 inch of the insulation on both ends. Secure one end on the R terminal where you removed the red wire from your thermostat. Take the other end and touch the G terminal on the same electrical board. Your fan will start. Don't worry about an electrical shock, the voltage is only 24 volts. Now with one end still attached to the R terminal, touch the W terminal where the white wire was attached. The furnace will now try to fire up. Hold it there until you see that it did start. If you did this and had success, I would say it was your thermostat or the wire going to it. Last resort would be a bad control board. If you want, you can do test your thermostat wiring by removing the thermostat from it's mounting plate. Put all wires back where they were on the furnace. Make sure the access door is back in place and secure. Take that same jumper and jump across the red and white wires on the mounting plate. Again the furnace should start up. Have a helper jiggle the thermostat wires where accessible when the furnace is running and see if it remains on or shuts off. If everything checks out, replace the thermostat. Good Luck!