If the burner blower is blowing, the following sounds should be heard. After about a 40 seconds after the burner blower starts, the gas valve should click and the igniter should buzz for a few seconds. If you don’t hear either of these sounds, the ignition module may be failing. The two yellow wires plug in to the ignition module at terminals marked 25V Ground and 25 V. Unplug these two wires and check across the two wires with a voltmeter with the burner blower running. There should be 25V AC present across the two wires. If a 25V reading is present on the voltmeter, the ignition module has failed and must be replaced.
*Refer to service department for exact model.
If the 25 V is not present across the two yellow wires and the burner blower is blowing, the centrifugal switch built in to the burner blower may not be working. This is a safety switch built in to the burner blower so the gas function will not work if the burner blower does not come on or does not reach normal speed. Sometimes this micro switch button cab be lubricated with light weight oil and get it to start working. Normally, the spring arms pull the white nylon cam away from the micro switch button and allow the button to move outward. If the button sticks in, the burner does not know the blower is running.
Remove the two nuts from the end of the cover and pull the cover straight off.
Move the white nylon cam to access the red micro switch button. Using a small screwdriver, you should be able to push the button in a slight amount. If it is stuck in and does not move try oiling with light oil like WD40.
If this is not successful producing the 25 V across the two yellow wires, the centrifugal switch is bad and the burner blower must be replaced.