In this month’s troubleshooting situation we find ourselves dealing with a gas furnace, and the customer has called to say that “there is no heat at all.”

This is a residential unit that is equipped with an LED readout system that flashes a fault code in the event of a breakdown in operation. In this specific situation, when you arrive approximately two hours after the customer’s call was placed, the LED is showing one short and three long flashes, and the unit is not operating. When you consult the fault code information shown on the inside of the furnace panel, the LED sequence you observed prompts you to do three things: (Refer to the accompanying diagram in Figure One)

  1. Make a visual inspection of the manual reset FRS (Flame Rollout Switch), which indicates that it is closed.
  2. Use an ohmmeter to test across the FRS terminals, which shows it as closed.
  3. Use an ohmmeter to test across the LS (Limit Switch), which shows infinity.

These checks lead you to further investigation, and the next component on your list is the blower motor. Proceeding with your testing, you find the motor cool to the touch, and after isolating all the wiring connected to the COM, LO, MED LO, MED HI, and HI wires, you find a reading of infinity when testing from the COM wire to any other wire lead of the motor.

Your two-part troubleshooting question:

  1. Which component failure caused the equipment breakdown and must be replaced?
  2. What is the secondary component that also must be replaced?

Let us know your answer here!

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