I've got a 1997 Lennox G20 furnace. A few weeks ago the blower stopped coming on after the burners turned on. The burners would come on for about a minute and then the furnace would shut down because the blower wasn't coming on. Though I could turn the fan on manually from the thermostat by setting the fan to 'On'. The diagnosis was a faulty control board. I replaced the control board with this: https://www.supplyhouse.com/ICM-Controls-ICM289-ICM289-Furnace-Control-Module And it fixed the problem for about a week and everything worked perfectly.

Today, I noticed the temperature on my thermostat was 3 degrees below what it was set to but it said it was heating, but the furnace was not running. I can no longer manually turn the fan on from the thermostat nor does the furnace ever seem to enter its heating cycle when the thermostat clicks on for heat. The furnace doesn't make the clicking sound it always makes prior to the burners coming on, the burners don't come on, it doesn't appear that anything is attempting to work. There is an LED light on a part called a continuous retry that is usually blinking green, but it is not coming on at all anymore.

What might the cause be? I don't see any loose wires, what could I have knocked loose that would be causing the symptoms?

I've got a voltage detector pen, the brown wire coming from the switch to the control board is live, I'm not sure what else I should be testing to see if it's live or not.

  • If you can provide a picture of the wiring diagram in the furnace, someone may be able to tell you where to put your meter to check for voltages that should signal the furnace to turn on. However, after 25 years, it may just be dead.
    – FreeMan
    Nov 21 at 14:52
  • Congratulations: you loaded the parts cannon and fired it at your problem, and it didn't work. I'll draft an answer for you shortly.
    – KMJ
    Nov 21 at 18:25
  • On the control board there is a 24V and two C terminals. With the blower door safety switch depressed, do you measure about 22-28 volts AC between the 24V and C terminals? If not then go looking for a fuse inside the furnace, or possibly a failed transformer.
    – Greg Hill
    Nov 21 at 20:10

2 Answers 2


To properly troubleshoot this problem, you're going to need to do a few things.

First is to confirm that you don't have something else in the wiring stopping the blower from working, such as a limit, safety, or pressure switch. You will need the service manual to complete this. First, look over the wiring diagram and use the continuity test on your meter to confirm that all limit switches are in the correct state, usually closed. If you find one that is open, it needs to be replaced after confirming that whatever condition opened it is no longer true. They do go bad, but you can't just assume it's a switch that failed and need to confirm that there's no underlying condition.

Once you've confirmed that limit switches are not your problem, go to the sequence section of the manual. Try to run a heating cycle and confirm what step the unit stops on. That will provide you with further troubleshooting steps. I bet the 'blower run' step isn't happening, based on what you described. Usually that's one of the last steps after flame is confirmed, to avoid blowing cool air out the vents, but that's not always the operational sequence.

However with all this said, since it's not running when you set the fan to On, I have a suspicion of what is going on here: your blower is worn out. Motors often take more power as they age. When you started having the control problem in the first place, you needed to put a clamp ammeter on the motor power wire to confirm that it was still within spec before replacing the control board. If it's been drawing too much power, it could easily have killed the previous control board, and has now killed the replacement. You can confirm this by looking up the blower amp specs, then jumpering it temporarily to power and measuring the current with a clamp ammeter.

  • If you don't have a continuity meter and a clamp ammeter, this will be harder to do. And as FreeMan pointed out, this unit is old. It's possible you will need a blower and a control board to fix this, and the heat exchanger might not be far behind. Your call if it's better to keep fixing it or to replace.
    – KMJ
    Nov 21 at 18:35

Thanks everyone for your input. This was too complex for me so I had an HVAC guy come take a look. It ended up being a blown transformer. We'll see how long it lasts. He didn't really have a good theory as to why the board would die and then a week later the transformer would die. Perhaps it's the old blower drawing too much power, but maybe just a coincidence.

  • Was the transformer on the same board ? If so, you are looking at something in your house being the cause. 22 hours ago

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