[c9340]

Have Goodman Furnace built 1994. Worked fine until about 3 weeks ago.
It starts cycling, and the heat icon does not show on the thermostat. I turn the thermostat off, the furnace continues to blow. Last night to verify, took the thermostat off the wall (but still had the furnace plugged in and at 4:30 this morning, it turned on) had to unplug to stop. If I raise the thermostat above the current temp, the heater will turn on. Could it be the Control Circuit Board... other ideas? Had service tech out, of course it wouldn't repeat, but he said 80% circuit board 20% thermostat...
When it begins it turns on for a couple of minutes, then turns off for a few more, then turns back on. Seems to do it mainly in the evening/early morning hours, we are not home during the day, but weekends it doesn't do it during the day. Thanks for anything you can suggest

[labman]

The blower is controlled by a relay and timers or limit switches. On a heat cycle, the gas comes on, and then blower after a time or a temperature is reached. After the gas goes off, again either time or temperature shuts the blower off later. The off delay could be bad, or there could be a problem on the board. Most blower problems are due to a relay problem, but usually it is a problem of the relay either staying open or closed, not opening and closing on its own.

One other possible problem is a short between the contacts on the back of the thermostat. It does have a back that stays on the wall when you pull it? Pull the back off and look for any possible way for power to get from the red wire to the green wire. Also check at the furnace when the blower is running. See it you have 24 volts between the green and blue contacts.

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need

[labman]

Digital or programmable thermostats may need the blue wire connected to them.

Likely a problem from me writing everything in NS 4.x and pasting it in. I am spelling typing impaired and haven't found anything to match its powerful and friendly spell check. The newer versions use a piece of trash copied from MS.