I've spent the last couple of days troubleshooting my Lennox G40UH furnace.
A couple of weeks ago, I noticed that the furnace would sometimes come on briefly (anywhere from one second to a couple of minutes), then shut down. But otherwise, it seemed to be keeping the house warm.
Recently, the problem has gotten worse. Sometimes, the furnace will light then shut off partway through the heating cycle. Other times, it will get into a repeating pattern where the inducer comes on for about 5 seconds, then the unit shuts down for about 90 seconds.
When the failure occurs, the unit shows alternating fast flash on the two indicator lights. The manual indicates a few possible causes:
- Bad ground
- Line voltage below 75 V
- Open, broken, or failed ignitor
I've checked the line voltage at the control board, and it's a steady 122 V throughout the heating cycle. When the problem occurs, there is no visible change in the voltage on my multimeter.
I've tried using an alligator clip to connect the chassis to a ground outside the unit. For a while, I thought this had fixed the problem, but it's not 100%, which makes me think the issue is elsewhere, but maybe made worse by a ground issue.
Removing the 4-pin connector from the control board and checking the resistance on the ignitor, I get 16 ohms, which is within spec (10.9-19.7 ohms).
If I disconnect the ignitor completely, the unit cycles between 5 seconds with the inducer motor on and 90 seconds with the error lights flashing, the same as the worst case noted above.
I tried labeling and disconnecting all the wire from the control unit (56L8401), gave the board a look (no obvious signs of damage), and put it all back together. After this, the furnace worked for a couple of hours, but then got into the 90 second on/off cycle again.
This morning, the furnace was stuck in the 90 second on/off cycle for a couple of hours. I thought I'd try connecting the chassis to an outside ground again, and this seemed to fix the issue (at least temporarily).
I'm leaning toward the ignitor as the cause here, but am surprised that (a) the ignitor always works perfectly if the heating cycle gets that far; and (b) sometimes connecting to an external ground seems to help. This makes me think the problem could be, e.g., with the control board, so I'm hesitant to spend $100 for a new ignitor only to have the problem persist.
Do these issues sound familiar to anyone? Are there any other tests I can do before replacing components, to be more sure that I'm replacing the right part?
Thanks!
Edit: I've verified that disconnecting all the thermostat wires and adding a jumper between R and W does not solve the problem.
I've also tried disconnecting the HUM-H and HUM-N wires from the control board. These go to the humidifier and are energized with the inducer. This hasn't solve the problem, but has made the most consistent improvement so far. The furnace now doesn't fall into the "5 seconds on, 90 seconds off" cycle. It will sometimes abort partway through the heating cycle, and show alternating fast flash on the two indicator lights, but this will only happen once or twice before the unit completes the heating cycle.
We did have an issue about a month ago (perhaps when these problems started) where the humidifier drain became blocked, and water leaked out of the unit. I wonder if this might have caused some issues within the humidifier, so that when it's plugged into the control board it unbalances the line voltage in a way that the control board sees as a bad ground.
