Lennox Furnace G40UH lights off

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Old 12-25-16, 03:17 PM
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Lennox Furnace G40UH lights off

hello,
my lennox g40uh had a open limit switch error from its diagnosis code (slow red flash and solid green). it would attempt to heat but stopped after a few seconds. To reset the unit i set the thermostat to off, turned off the circuit breaker switch and close the gas line. i turned everything back on starting with circuit breaker, then gas line, then thermostat to 10 degrees above room temperature, but the system still appears to be off. my diagnosis LEDs (red and greed) are both off. so i turned everything off again, replaced the air filter and limit switch (OEM part), turned everything back on (same sequence as above), still have an off furnace. the diagnosis LEDs are also off.
any help would be appreciated.

the exact model of my furnace is Lennox G40UH-36A-070-15
thanks,

M
 
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  #2  
Old 12-25-16, 04:31 PM
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According to the manual: https://www.questargas.com/ForEmploy...X%20SERIES.pdf

Both LEDs off indicates a control board failure or wiring error.

Things to check:

Make sure all access panels are properly installed on the unit so the safety switch(s) are not open.
Make sure you are getting 120 VAC to the unit.
Make sure the internal fuse isn't blown and that you have 24 VAC at the secondary of the transformer.

When starting up the unit, the gas should be turned on before the power, but that shouldn't be causing your problem.
 
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Old 12-25-16, 05:14 PM
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I'm guessing the blower door was left off. There should be a sight glass in the door
so that you can see the LED's with the blower door in place.

Any brief interruption in power will reset the board.
 
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Old 12-26-16, 10:52 AM
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Hello again,
not sure what you mean by the blower door. i just removed the front panel and placed it back once the limit switch was swapped out. as for wiring, i only plugged out 2 wires which plugged into the limit switch. the connectors each have a unique adapter so i know i have it connected properly. the orientation of the limit switch is correct as well. its strange that the board gave me diagnosis before but now wont turn on after a circuit breaker reset. that shouldn't break the board, how else was i going to cut the power to the furnaces, there is no switch on the side of the furnace. where do i check to see i'm getting 120 VAC? anything else i can look for, shouldn't the system just come back on after a reset? am i missing something here?

thank you for your feedback.


M
 
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Old 12-26-16, 06:12 PM
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update on my furnace

ok, after further investigation, it turns out i had a open fuse on the board. after replacement, the furnace came on. but, it when back to my original problem which was, "Primary/secondary limit switch open or Watchguard mode-limit switch open longer than 3 min"

i've already replaced the limit switch. not sure what to do next. any advice would be appreciated.

thank you,


M
 
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Old 12-26-16, 06:31 PM
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Primary/secondary limit switch open Lennox g40uh furnace

hello,
my lennox g40uh has an open limit switch error from its diagnosis code (slow red flash and solid green).
i have replaced the air filter, and the limit switch and reset the system but am still getting the same error code.

full error message from
Primary/secondary limit switch open or Watchguard mode-limit switch open longer than 3 min"
https://www.questargas.com/ForEmploy...X%20SERIES.pdf

the exact model of my furnace is Lennox G40UH-36A-070-15
thanks,
 
  #7  
Old 12-26-16, 06:39 PM
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Which limit switch did you change? There are two flame rollout sensors, a gas limit switch, and a combustion air proving switch.

The flame rollout sensors may have a manual reset button on them.

In any case, you need to use a multimeter to find out which limit is open before you can figure out what might be wrong. Most of the time, it is not a switch/sensor problem, but another problems that is causing the sensor to trip.

When you say it would attempt to heat, exactly what does the furnace do when you turn the thermostat up?
 
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Old 12-26-16, 08:28 PM
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update

still getting error code: "Primary/secondary limit switch open or Watchguard mode-limit switch open longer than 3 min"

this is what i've done so far:
air filer replaced
primary limit switch replaced
flame sensor cleaned
tested flame rollout switch, they are closed (good)
secondary switch closed, (good)

after i reset the system, the flames and fan turn on to meet thermostat request for more heat. after about 3 minutes, the furnace stops fan and flame and gives the error code stated above (slow red flash, solid green).

any ideas?

thanks,

-M
 
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Old 12-26-16, 09:42 PM
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Hi Paul,
i replaced the primary limit switch. i do have a multimeter and i did test out the 2 flame rollout switches. they were both closed. i dont see a gas limit switch on my diagram (page 3 https://www.questargas.com/ForEmploy...X%20SERIES.pdf)

i have cleaned the flame rollout switch with a steel wool. i also have a new air filter in there.

after i reset my furnace, my furnace fan turns on along with the with my 3 flames (thermostat is set to ON, about 10 degrees above room temperature). the diagnosis lights (red and green flash fast simultaneously, which means "Normal operations - signaled when heating". this goes on for about 3 1/2 minutes, then fan and flame stop and diagnosis light go to slow red flash and solid green which means "primary/secondary limit switch open or watchgarud mode limit switch open longer than 3 minutes.
 
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Old 12-26-16, 09:53 PM
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Is the main air blower failing ?
If it isn't then there is something else impeding furnace airflow...... like dirty filter, clogged A coil, too many registers closed down, blocked return vents.
 
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Old 12-26-16, 10:05 PM
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I have combined your two threads into one. It's better to have all the info in one place.

The rollout switches are not your problem. They don't get cleaned. They will trip requiring a manual reset.

The limit switch that is opening is the one that monitors the heat exchanger temperature. You can put your meter right on the sensor while it's in operation. Set the meter to 24vac scale. When you see 24vac..... the switch has opened.... or tripped.
 
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Old 12-27-16, 03:38 PM
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i can see the inducer motor turning. i can also hear the blower motor.
my registers are open. i check the vent the leads to outside and it appears clean.

i'm still getting error code: "Primary/secondary limit switch open or Watchguard mode-limit switch open longer than 3 min"

this is what i've done so far:
air filer replaced
primary limit switch replaced
flame sensor cleaned
tested flame rollout switch, they are closed (good)
secondary switch closed, (good)
made sure the registers gates are open.
checked and see inducer motor turning and can hear blower motor.
can see all my (3 of them) come ON.

after i reset the system, the flames and fan turn on to meet thermostat request for more heat. after about 3 minutes, the furnace stops fan and flame and gives the error code stated above (slow red flash, solid green).

this is where i'm stuck. would next step be removing the blower motor and cleaning it? any recommendations for next step?
 

Last edited by mehdi0806; 12-27-16 at 03:40 PM. Reason: more info
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Old 12-27-16, 06:09 PM
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As PJmax said, you need to figure out which sensor is opening. It could be the Primary limit or the secondary limit (looks like your model has 2 of them).

Put your meter, set to ac volts, across each of those sensors, one at a time and cycle the furnace. If a sensor opens you will see about 24 volts across the sensor (briefly).
 
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Old 12-27-16, 07:31 PM
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I connected a multimeter and set it to ac reading. The reading was zero until the primary limit switch opened (this is a new switch) then it jumped to 27 volts. It took about 3.5 minutes to open after i reset the furnace and the flames came on. So we know the primary limit switch is opening but why? What's the root of the problem? I listed all the work I've done in my last post. Any suggestions for next steps. Thank u all.
 
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Old 12-28-16, 06:15 AM
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Since you replaced the switch and it still trips, that tells you the furnace is overheating.

That usually indicates reduced air flow through the furnace. You've changed the filter and made sure all registers are open. If the furnace also has air conditioning you should remove the access panel covering the A coil for the A/C and see if the coil is dirty and restricted.

Also examine the heat exchanger for obvious cracks or holes.

Manually turn the main blower wheel (not the inducer) with the power off and make sure it turns freely and smoothly. Make sure the blower wheel isn't all covered with dirt.

Remove the door to the blower compartment, temporarily tape the door switch down to allow running the furnace with the door off, and try running the furnace. This will rule out restricted return air supply.
 
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Old 12-28-16, 08:25 AM
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I pulled out the blower (not the inducer) and it was relatively clean, I could easily spin it with my hand. I put it back in, kept the door covers open, taped down the door switch and ran it again, but still have the same problem. After about 3.5 minutes the primary limit switch opened. Is there anything else to check before the heat exchanger because it's not easy access? Thx
 
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Old 12-28-16, 12:24 PM
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Try running the heat with the thermostat in FAN ON mode and see if that keeps it running.
FAN ON should be a faster motor speed.
 
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Old 12-28-16, 03:28 PM
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Yeah, I've been running the thermostat on ON not auto. The inducer motor is going pretty good. I'm going to look at the blower motor one more time, this time make sure it's spinning. It's hard to say if through sound I hear is from the inducer motor or the blower motor. I'll check it out tom8ght and will report back. Can a bad blower motor cause the primary limit swtch to open?
Thx
 
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Old 12-28-16, 04:56 PM
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Absolutely! If the blower isn't running your furnace will overheat very quickly.

Just turn the thermostat way down but set fan to on. Then the blower will be the only thing running.
 
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Old 12-28-16, 07:16 PM
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Yeah it appears to be the blower. Do I first replace the capacitor and rerun or just bite the bullet and buy a new motor?
 
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