heat won't turn off

I have a gas furnace. When I woke up this morning it was about 80F in the house, even though the thermostat was set at 68F. After turning the thermostat all the way down the heat was still blasting and I had to shut the gas valve on the furnace to shut the thing down. Before I call & heating & ventilation specialist and pay big bucks, might there be a way to troubleshoot this on my own? I'm wondering whether I simply need a new thermostat and, if so, whether I could check this and install it on my own. Any help would be greatly appreciated.
W.D. (aka, sweating in Wisconsin)
ps -- the temp hasn't been abnormally cold -- in the 20s
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Disconnect thermostat. Is the heater still running? If not, replace thermostat.
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I would say if the furnace still runs with the t-stat disconnected, to disconnect the t-stat wires at the furnace, as It could be in the wiring from the t-stat to the furnace. Tony

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Electronic programmable thermostats will sometimes malfunction and do this. You may be able to reset it if you have the instructions. Don Young

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