I have a XE80 furnace (installed sept.1995) the light blinks five times. The thermostat is off but a fan motor comes on that can be heard at the return vent.

On my unit this is a flame rollout sensor error. Check your rollout sensors there will be a couple in the burner area they are small and round and have two wires going to them, some are resetable and have a small red button. Check for continuity through the sensor with a multi-meter. If they are open push the red button or replace them if they are not resetable. If you trace the wire you will see it is daisy-chained to make the cisrcuit through all the sensors, follow it as there may be other sensors depending on your configuration. You should be able to trace it from the control board in/out of all the sensors and returning to the control board. There may also be one in the fan blower box that will require you to remove the control board and flue vent pipe in order to pull the panel accessing the fan blower.