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.