Service & Maintenance / Extra Edition

Troubleshooting Challenge: A Gas Furnace That Won’t Fire

In this situation we have a customer who has called for service on their gas furnace, and the description of the problem is “no heat at all.” The customer also reports that this is the first time they’ve had to call for service on their 10-year-old unit. When you arrive and start your troubleshooting procedure by checking to make sure that the control system is set properly, you find that nothing happens when you select the heating mode on the thermostat and set it above the room temperature.

As your next step, you confirm that the thermostat is not the problem, and you then locate the wiring diagram (see Figure 1) and determine that the method of operation for this unit is to confirm the operation of the vent system before allowing the hot surface igniter to light the burner. Determining that the induced draft motor (IDM) is cool to the touch, you locate the quick-connect for the IDM, you position your voltmeter at 1 and 3 on the connector (black and white), and when you again implement a call for heat, you read 120 vac.

As your next step, you isolate the IDM and note that it is cool to the touch. When you test between the connector points 1 and 3 on the motor wiring harness with an ohmmeter, your reading is infinity.

Your troubleshooting question: What is your next step in getting this furnace back on-line?

Compare your answer with ours by clicking on the PDF link below.

Publication date: 11/07/2011

PDF - November Troubleshooting Answer

For information on Jim Johnson’s HVACR technician training DVDs, go to

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