It's a Goodman GUPI100-4. Everything appears to work right until the furnace blower kicks in. Then the burner shuts off and the blower fan continues to run until I guess it cools down to its limit. After a minute or two it goes through the same cycle. HELP.:)

Does the LED on the circuit board start blinking when this occurs? If so, observe the # of blinks, then check service label for description. If not, try this: disconnect wires from "R" and "W" at thermostat and connect them together. If normal operation is restored, the problem is in the thermostat.

No LED on circuit board. Connected the two wires and it worked great. Will replace thermostat tomorrow and let you know. Thanks for the great help. For my info... what did the connection of the two wires do?

Apparently, the thermostat can't handle the gradually increasing load of the control circuit, and activating the blower relay is the proverbial "last straw", causing the control voltage past the thermostat to drop. Either the anticipator, selector switch, or "W" relay at the thermostat is bad. By connecting the wires together, you bypassed the thermostat so the unit could reach full operation. Good job.

After disconnecting the old thermostat and before connecting the new thermostat I would jump the R or RH and W wires at the thermostat wire end that goes to the thermostat and check furnace for proper operation. This way you will also know if the thermostat cable going to the furnace is OK before hooking up your new thermostat. An extra 2 minutes work can save a headache after your new thermostat is hooked up.

Read this, may apply. Combustion/ Ventilation Air Supply Sizing Let me know if applies.

Replaced thermostat... works like a champ. Thanks KC-13.

Glad to hear it. Playing "HVAC HERO" on PS-3 is really paying off... :D