My furnace runs to operating temp on thermostat shuts down then within 30 seconds try's to cycle back on three to four times. What would cause this.

That's how it should work .Your furnace will cycle on and off to reach set point .
You don't have to troubleshoot the thermostat. The reason why your furnace cycle is once the heat exchanger get to a certain temperature the board will shut it off, fan would still work to cool heat exchanger down then once cool down furnace will fire up again .

If you have a switch that can manually turn on the fan, try it to verify the fan works. This is what I think is happening. The normal operation should be thermostat calls for heat, inducer fan comes on, pressure switch closes, igniter glows, main burner ignites and then in 30 or so seconds the fan comes on and stays on until the thermostat is satisfied. Once the thermostat is satisfied the burner shuts off and the fan runs for approx 120 seconds then shuts off. I think the control board that automatically turns your fan on is faulty. On most modern furnaces if the auto fan control fails and the furnace gets to hot and limits, when the limit opens and shuts off the burner it also energizes the fan motor to cool the heat exchanger.

If your furnace is relatively new, as in 10 years old or less, there should be a field adjustable 'cool down' cycle. On the circuit board there should be a dip switch or series of dip switches with 30, 60,90,120 indications. These numbers represent the length of time the relay on the circuit board allows the fan to run AFTER the thermostat satisfies. You may be able to adjust the time frame down one position from where its positioned now.


The second possibility, if the blower never shuts off at all is that the relay itself could be stuck in the closed position and keeping the fan on.

check that the replaced thermostat is the same temp degree setting of the last one

On the call for heat from the thermostat the inducer should come on first, a pressure switch proves that the inducer is moving enough air up the stack, closes and energizes the ignition and burners come on. 30 to 60 seconds later the blower comes on. Everything should stay on as long as the thermostat is still calling for heat. When the thermostat is satisfied the burner and inducer shut off and the fan will run to remove the heat from the heat exchanger and then shut off. Cycle complete. Some inducers have a post purge so when the thermostat is satisfied the burner shuts off, the inducer runs for 30 seconds or so and then shuts off. I am thinking that your furnace may be running on the limit switch and when the burner is off but the inducer stays on, maybe the limit is shutting of the inducer. Check your air filter and make sure all your heat registers are open and that the return air grill is not blocked.

On a gas dryer the heat normally only comes on for 30 seconds or until the drum temp reaches 160f at which time the flame is shut down by the cycling thermostat. The drum temp then starts to drop. When the drum temp reaches 105f then another blast of heat is issued for 30 more seconds.That is how they work. Put it all back together and trow a load in there.


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Sandpaper lightly scrubbed over the sensor tip to remove any contamination.

Sometimes cleaning is not enough and the sensor must be replaced.

Could be a defective thermostat in the plenum.
When a gas furnace shuts down the fan(blower) will run until the plenum cools to the safe range. Usually 1 and 1/2 minutes.
Intermittent operation is a defective thermostat or relay.

Joss sounds like the anticipator setting on thermostat i would replace it with a digital it will be more actturat . The furnace has to go over temp setting to turn off

A Make and Model number of the furnace not the thermostat will help but I can walk you thru the operation of a typical furnace. It sounds to me that the burners are igniting properly but the flame sensor is not sensing the flame and shutting down the system. You will need to find the sensor and clean it and check the wire from it to the control board. Typically, the flame sensor is found at one side or the other of the burner box. It is a small probe that the flame hits. Depending upon your model it may have a small wire attached to it leading back to the control. Or it may be part of your auto-pilot assembly. ? At any rate, you will need to get to it and very delicately brush it off. Then inspect the flame channel. This is a trough area where the flame travels to ignite all burners. Over time, they tend to fill with rust and debris. Take a small screwdriver and run the blade down the trough. Inspect the wire that goes from the flame sensor to the control board. Make sure it is not frayed or broken or shorting out and that the connection at the board is good.

Beyond that, it's time for a technician to come.

Let us know the Make and Model of the furnace (found inside the furnace cabinet) and maybe we can help you further.