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Lets start with letting me get a few more details.
1.If you turn thermostat to OFF does it still heat
2.Model on thermostat
3.Model on furnace
OK if heating in even the off position lets start by determining if issue is in thermostat or not.
Disconnect all thermostat wires from thermostat and see if it goes off. We will go from there.
OK, So that tells us thermostat is the problem.
Tell me where each wire was going on stat (EX Red to R, Green to G, etc)
OK now put them back one at a time starting with red and tell me when furnace starts
What is on heat?
Have you put all wires back one at a time?
Ok so that is good news, All wires are connected now and furnace is not on YES?
OK now raise temp setting up about 4 degrees above room temp and see if heat comes on
Now lower and see if it will cycle off
Give it about 3 minutes and make sure numbers are well below room temp
Burners and fan both still on?
Hate to say but you have proved issue is in tstat if furnace went off with tstat disconnected.
Just to be sure nothing else we can do, make sure the E/ F on thermostat is SET to F and cycle rate is set to Switch 1 and 2 both OFF
There is no such relay on furnace as it is controlled by a circuit board and even if relay was stuck, disconnecting wires at thermostat would not unstick it.
Let me know hat you figure out.Curious to know as the Honeywell is usually pretty reliable.
I still say if you disconnect thermostat wires form furnace and unit shuts off issue has to deal with thermostat unless it was coinidental
Let him know it stops when disconnecting thermostat