Furnace wont start when it should

[aboyer]

I have an Armstrong Ultra 5 gas furnace. This problem just started yesterday... My digital thermostat is set to 73 degrees but the furnace won't kick on till the temp goes well below 68... If I get impatient/cold I turn off/on the thermostat (just put in new batteries)and I hear it click, but no furnace. I then go down to furnace and turn off/on the power to it and it immediately starts.

It will also shut down before it reaches the desired temperature. I checked and the thermostat still appear to think its on....

What would cause that??? I am assuming some type of sensor.

[aboyer]

A little more info... the unit has a Honeywell smartvalve that is blinking an error code when it isnt working... 6 flashes then 4 flashes. Trying to track down what it means...

[ecman51`]

Before I took too many steps, I'd want to first find out what the stat was doing, down at the furnace hookups for the stat wires, with my voltmeter.

[aboyer]

All is well... I feel kinda foolish but... 6+4 flashes means lockout due to limit switch. This typically occurs due to obstruction of airflow. It turns out between my wife and children they completely blocked off 2 of my 3 cold air returns. No air flow caused the furnace to overwork and thus trip the limit switch.