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Heating, Air Conditioning, Fridge, HVAC/Lennox furnace G32Q3-75-2 problem


QUESTION: Hi! I have a problem with my Lennox furnace G32Q3-75-2. It's about 10 or so yrs old and has had this problem for two yrs now, intermittently. It will be working fine, then stop putting out heat. The motor still runs, but the diagnostic code is D1 off, D2 slow flash. The manual says this is due to pressure switch open, or has opened 5 times during a single call for heat. Or, blocked inlet/exhaust vent. Or condensate line blocked. Or pressure switch closed prior to activation of combustion air blower.

Dring a recent tune-up a few weeks ago, the tech noticed water backed up in a small vacuum hose going to the exhaust. He pulled the end off and blew it out. He also removed a rubber trap and emptied it. It had been working fine until today. It stopped heating, LED lights gave same code. I removed hose and blew out water, removed trap and it was full. I reset the power switch, and it worked. Ten a few hours later it stopped again. Same symptoms. I again found water in the line and in the trap. But when I removed the water and power cycled the furnace, it went back to the same diagnostic code.

There is no apparent block in the inlet or exhaust pipes outside. The blower is working. It is -5F right now, so pretty cold.

Any ideas?



ANSWER: That is a problem getting that much condensation build up in the unit.  I know from time to time, but your case it is excessive.  Do you know if during the tune-up a flue gas analyzer was used?  Improper combustion can cause the problem.  You may even have air leaks or problems with the flue pipe.  

It is possible the water damaged the pressure switch.  You might try to cycle the power again.


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QUESTION: I'm pretty sure they tested the combustion during the tune-up, but can't seem to find the checklist they left me.

Figure 18 in the manual shows the tubing exactly how mine is arranged - with the tubing going under the inducer motor and connecting to the trap, slightly above. The water seems to collect in the portion of the tubing under the inducer - which would be expected since it is below the level of the trap. Am I missing something?

I think the pressure switch(es) need to be replaced by process of elimination. Is there an easy way to test them?

ANSWER: When I answered the question last time, I thought you were referring to the tubes that run from the fan to the pressure switch.  The drain line I believe you are referring to should have some water in it, along with the trap.  You just need to make sure the drain line that is external of the unit has free flow.

You can use a meter to make sure the switches are open when it is off.  You can also use the meter to see if the switches close when the fan turns on.  To truly test the switches, you have to have a manometer or a magnehelic guage to test the pressure so you know that there is in fact proper air flow and the switches are opening at the proper pressures.  Table 12 in the file you set me as the proper settings for the switches.

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QUESTION: Sorry for the confusion. Yes, the water is in the drain line. There is no water in the inducer motor or any of the tubes going to the pressure switches. They are all clear and dry.  

Well, I just got the two new pressure switches delivered. I hooked them up, and the same diagnostic signal occurs after the call for heat from the thermostat - the diag light #2 slow flash, ie pressure switch open, or has opened 5 times during a single call for heat. Or, blocked inlet/exhaust vent. Or condensate line blocked. Or pressure switch closed prior to activation of combustion air blower.

If I detach the hose from the inducer motor, the error clears and the furnace starts up and runs properly until the next cycle, when the same thing will happen again. There is no blockage in the port where the tube connects to the inducer motor. Any ideas where to look next? Why does it come on when I detach the negative pressure hose from the inducer motor?


With the assumption that EVERYTHING is perfect with the unit, I would have to guess the control board has gone wacko.  The problem lies in the fact the issues has to be duplicated to really figure out what is wrong.  Sometimes it just can't or everything else that could be an issue has been eliminated.  Control boards can do strange things that can drive a person nuts.  

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Craig HVAC Expert


I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.


I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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