Service & Maintenance / Extra Edition

Troubleshooting Challenge: A Gas Furnace That Won’t Heat

In this troubleshooting problem, the customer’s general definition of the situation gets more specific when you arrive at the site and discover that the reason there is no heat is because although the burners ignite on a call for heat, the air handling system never starts, and the system’s limit control shuts the furnace down. With that determined, you remove the access panel, consult the equipment diagram (click here for a PDF of the diagram), and take the following steps, which yield the results listed.

With a voltmeter attached to PR2 and TP3 (Test Point 3) on the board, initiating a call for heat proves that the IDM operates, the hot surface ignition system functions, the burners ignite, and the system shuts down with 120 VAC shown at your meter test points.

A subsequent check at the COM and MED LO connections at the motor also shows 120 VAC on a call for heat, and the system again shuts down.

Ohmmeter tests at the motor PL5 connector shows the following:

• From COM to HI…………9.5 ohms

• From COM to MED HI…..7 ohms

• From COM to MED LO….Infinity

Your troubleshooting question: What is the next step you will take to get this unit back on-line?

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

Publication date: 1/7/2013

PDF - January Troubleshooting Answer

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

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