My 15 yr old Bryant 394GAD gas furnace started this fall by doing everything OK except that when the burner clicked off, the blower would cycle off and on 2-4 times before shutting off completely. Now the problem has progressed to where the blower won't shut off at all after the burner is shut down unless I turn off the main power to the furnace. Jumping the limit switch does nothing as does disconnecting the green wire from the thermostat. Circuit board problem? Are there any troubleshooting things I can do to pin the problem on one part or another

Does it do this in heat only

Yes. Only on the heat cycle

We have some excellent HVAC people here. I wish the ones that know more than I do were answering more questions instead of the ones that know less. Maybe Northern Heat and the other good ones are all out on service calls.

In response to your email, I dug up your question. Does you furnace have an LED that flashes a code if there is problems? If it is the limit switch, you should be getting a code. If not we may be looking at stuck relay on the board or a timing circuit. The off and back on puzzles me. Usually a relay either stays stuck or works. Unfortunately replacing the relay means a $300 board. The board is a pain to trouble shoot too.

The blower uses a separate motor winding for heating than A/C or fan on, so the green wire has nothing to do with this case.

If the burners cycle on and off by turning the thermostat up and down but the blower keeps running the blower heat relay is sticking or the blower timedelay off is going bad. If a limit switch is open the burners will not ignite and the blower runs all the time. I think that furnace had the HH84AA020 board and was known for sticking relays. They have a new style of board that will replace it.