I have a Trane XV80, model TUD100R9V5H6, Serial #P303RDH2G. This weekend I noticed the fan running constantly while the heat/air was off. Unplugged the power to the unit for five minutes to allow it to recycle. When power restored, same problem returns. Checked the circuit board and saw the red flashing light warning or trouble indicator was cycling. Five (5) flashes, which means "flame sensed when no flame should be present". Does this mean a bad circuit board or the flame sensor needs to be replaced. The unit is 10 years and has been problem free. All lights are cycling on the circuit board. No heat when the heat is set on the termostate. What can you recommend? A new sensor, circuit board? Thanks.