In this month’s troubleshooting situation, we find ourselves dealing with a condensing furnace, and the customer has called to say that “there’s no heat at all.” It’s a relatively small unit, rated at only 40,000 Btu, but it is equipped with an LED readout system that flashes a fault code in the event of a breakdown in operation. In this specific situation, you find that when you arrive at the customer’s home, this unit (a Carrier model) is flashing one short and three long flashes.

When you consult the fault code information shown on the inside of the furnace access panel, the LED sequence you observe prompts you to do three things (refer to the accompanying diagram in Figure 1):

1. Make a visual inspection of the FRS (flame rollout switch) which indicates it is closed.

2. Use an ohmmeter to test across the FRS terminals, which shows on your meter as closed.

3. Use an ohmmeter to test across the LS (limit switch) terminals, which shows a result of infinity.

These checks lead you to further investigation, and the next component on your list is the PSC-operated blower motor. Proceeding with your tests, you find the motor cool to the touch, and after isolating all the wiring connected to the COM, HEAT, COOL, SPARE 1, and SPARE 2, you find a reading of infinity when testing with an ohmmeter from the white wire to any other color wire lead to the motor.

Your troubleshooting question:

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Publication date: 11/03/2008