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

Did you enjoy this article? Click here to subscribe to The NEWS Magazine

Recent Articles by Jim Johnson

You must login or register in order to post a comment.



Image Galleries

2015 NCI Annual Summit

The National Comfort Institute held its 2015 Summit in San Diego. Posted: April 1, 2015.


NEWSMakers: Andy Patron
Andy Patron, partner with FMI Corp., discusses the impact a national labor shortage may have on the mechanical contracting industry. Posted on March 27.
More Podcasts


NEWS 03-30-15 cover

2015 March 30

Check out the weekly edition of The NEWS today!

Table Of Contents Subscribe

Global Warming

Do you believe in man-made global warming?
View Results Poll Archive


2015 National Plumbing & HVAC Estimator

Every plumbing and HVAC estimator can use the cost estimates in this practical manual!

More Products

Clear Seas Research


Clear Seas ResearchWith access to over one million professionals and more than 60 industry-specific publications, Clear Seas Research offers relevant insights from those who know your industry best. Let us customize a market research solution that exceeds your marketing goals.


Magazine image
Register today for complete access to Get full access to the latest features, Extra Edition, and more.


facebook icontwitter iconyoutube iconLinkedIn i con