Originally Posted by TG Twinkie
It's not that the plenum is hot. It's that the temperature sensor in the plenum thinks it is. The only time the room thermostat will stop and start the fan in direct response to the thermostat is when the plenum is cooled and the temp switch is satisfied that the plenum is cool enough for the fan to shut down.
Just for informational purposes, there is no temperature sensor in the plenum. There is an ECO, or emergency cut-off sensor, which will shut the furnace in an over-temp, over heating situation. The only sensor responsible for shutting down the furnace during normal operation is the wall thermostat.
In older furnaces, there is a TDR (time-delay relay) that is responsible for the purge cycle after the burner has extinguished after the t/stat has been satisfied. In newer models (such as the one under discussion), the igniter board has this job. It is also responsible for the blower coming on, checking the circuit thru the sail switch and ECO, igniting the burner and then extinguishing it and then controlling the purge cycle.
This board also tries 3 times for ignition and will shut down the blower if this doesn't occur. His igniter board probably needs to be replaced.