In this troubleshooting situation we have a residential customer who has called for service, and their explanation of the problem is that their six-year-old gas furnace “won’t run.” They also tell the dispatcher that they have already replaced the thermostat, but it didn’t solve the problem.

When you arrive, you find the furnace sitting idle, and the customer explains that since they were able to determine that there was power to the furnace receptacle by plugging in a desk lamp, they assumed that the problem was with the thermostat.

Upon removing the access panels, and manually initiating the Interlock Switch, you locate the pictorial wiring diagram shown in Figure One, and get the following results with your voltmeter:

…0-Volts at COM and 24VAC on the Furnace Control

…115-Volts at L1 and N at the black and white transformer primary connections

Figure One


Here is your two-part troubleshooting question: Which component needs to be replaced in order to get this furnace operating again, and what steps do you take to ensure its proper operation?

Until next week…

Learn from yesterday…..Live for today…..Look forward to tomorrow


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