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Coronach 11-11-2011 02:15 PM

Lennox G43UF Pressure Switch/Inducer Motor question
OK, so, I have a Lennox G43UF-48C-110-08 furnace that is not lighting off. The error code being displayed is Red LED off and Green LED at slow flash. This translates into:

1. Pressure Switch Open
2. Blocked air Inlet or Outlet
3. Condensate Line Blocked
4. Pressure Switch Closed Prior to Operation of Air Inducer

What I've done so far:

1. I checked for a buildup of water (blocked condensate line). The inside of the unit is bone dry.

2. I checked the intake and exhaust lines as best I can without disassembly. Neither seems to be blocked, but the vent line is not a straight shot from the furnace to the outside, so it is possible that there could be an obstruction in an elbow somewhere. Still, they seem clean, and the furnace has worked recently (so its not like a bird built a nest in there over a season of disuse).

3. I checked the air lines to the pressure switch, and neither one is blocked.

4. I followed some instructions posted elsewhere and tried to clip the electric lines going to the pressure switch together to imitate the switch closing, and thus triggering the draft inducer to fire. Nothing happened, though I'm not positive of the exact point in the sequence that I should close the circuit.

Edited to add:

5. The housing for the draft inducer motor is VERY hot to the touch when the furnace is getting calls to heat. Nothing else in the assembly to at all warm (since it completely fails to fire)

Can someone advise how to proceed? At this point, I'm inclined to think that it is either the P/S or the draft inducer motor. If my previous test was valid and successful, it sounds like it is the draft inducer motor. That said, I'm not sure I ran the test right, so it could still be the P/S, though if that's the case I'm not sure why the Inducer Motor is so warm.

Any advice? I'd rather not do a service call. My heatpump is holding the temp just fine, so I have a couple of weeks to screw around with this. And yes, I did make sure that the furnace was getting the heat call, and not the heat pump.



yuri 11-11-2011 02:18 PM

If the draft inducer is not running then you have no draft to close the pressure switch. Sounds like the circuit board is the culprit.

Coronach 11-11-2011 02:30 PM

Why the circuit board and not the motor itself? Also, did you see the edit I made at the same time you posted? The induction motor is very hot to the touch, but it does not spin (or, I have never observed it doing anything I would term "spinning"). Unless it is very quiet or it only runs when I'm not there, it's not running...though it could be the P/S telling it not to or the circuit board doing the same- if so, then why is it hot?



NitroNate 11-11-2011 02:42 PM

can't you bypass the pressure switch and then test? should be two wires running to the switch, just touch them together. then you can test the voltage at the white and black power to the inducer motor. there should be a relay on the board the kicks on the inducer motor.

i'm assuming if the motor is hot it is getting voltage and just not kicking on. could be the motor is shot, could be bearings, not sure. does the inducer fan spin freely when you spin it by hand?

Coronach 11-11-2011 03:01 PM

Bypassing the pressure switch is what I tried before, though I didn't try to test voltage. I'll dig out my multimeter and give it a shot when I get a chance.

I ran downstairs and tried to spin the induction motor by hand. Without removing the assembly, the only thing I can reach are the fins inside of the metal housing of the motor itself. I took a bic pen and used it to push on the fins to see if they would rotate. The moved with a little resistance, both clockwise and counter-clockwise. By a "little resistance" I mean that they were easy to turn by hand, but it wasn't like you could start them moving at watch the motor spin freely and then slow down of its own accord. It would move a little and stop right away, but as near as I can tell I spun it through a full revolution both ways, and the resistance seemed constant (not like a fan blade was hitting an obstruction at one point).


beenthere 11-11-2011 03:07 PM

If the inducer motor is hot, but not running, it usually indicates a problem with the motor, not the board. As the board is sending voltage to the motor or it wouldn't get hot.

DO NOT bypass the pressure switch on any furnace that the inducer is not running. The lives you save may be that of your family.

Coronach 11-11-2011 03:10 PM

Yeah, bypassing the switch was for testing/troubleshooting purposes only, not use. Right now the heat pump is doing the heating and cooling and the furnace is shut down. I'm not a big fan of carbon mono- or dioxide.


NitroNate 11-11-2011 03:12 PM

the bypass is for diagnostic testing only, however sounds like the motor is the culprit. the high limit switch saved my ass a couple years back when the control board fried and the blower failed to kick on after the set delay.

beenthere 11-11-2011 03:24 PM


Originally Posted by NitroNate (Post 768725)
the bypass is for diagnostic testing only, however sounds like the motor is the culprit. the high limit switch saved my ass a couple years back when the control board fried and the blower failed to kick on after the set delay.

Bypassing the pressure switch when the inducer isn't running performs no diagnostic test. It is just dangerous.

The inducer must be running or the gas will come out in a lazy manor and not ignite across all the burners, and when it finally does ignite, the excess gas will go BOOM.

Yes, i am fully aware of how to check a pressure switch by bypassing it. But it MUST never be done on a furnace when the inducer is not running.

NitroNate 11-11-2011 03:33 PM

well, i guess if you are diagnosing/testing/messing with your own system and you don't know enough to allow gas to start flowing and eventually go boom in your face, then you become a darwin award.

yuri 11-11-2011 03:43 PM

I did not see the edit in time. Sounds like the inducer is seizing. The impeller inside broke on a few and there was a recall on some G51 inducers. Finished now. Jumping the pressure switch won't tell you anything and can be dangerous and damage the heat exchanger and cause an explosion.

diyorpay 11-11-2011 09:39 PM

Some ID fan motors need maintenance with a few drops of light oil in 2 places. This lubes the bearings which could seize without it. Small input tubes near the top of motor so oil drains down.

I've replaced the entire assembly twice and can state that some of the motors need this maintenance while others are permanently lubed.

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