LENNOX model G40UH won't light

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Old 11-11-15, 09:15 AM
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LENNOX model G40UH won't light

This is about furnace of LENNOX model G40UH-36A-070-02. The gas burning canít turn on.

When the power just turn on, the pressure air pump just work for less than 10 seconds and stopped. The main air blower fan was keeping on.
The error code is: Primary/secondary Limit switch open.
I test the connection between first and second limit switch. Both switches are on and resistance is less than 1 ohms.

I keep the Primary/secondary Limit switches circuit open. Then power on, the pressure air pump keep active. And, the main air blower fan keeps on too. I believe the pressure switch works fine.
The error code is: Open Rollout switch circuit.
 
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Old 11-11-15, 09:29 AM
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Welcome to the forums.

The rollout switch(es) is located just above the burner area. They should have a little red reset button on them.

If you have a tripped rollout switch that requires immediate servicing. It could be dirty burners or even a heat exchanger crack.
 
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Old 11-11-15, 08:29 PM
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Sorry. I checked the manual. Looks my problem is still the Primary/secondary Limit switch open issue. I measured the primary limit switch and looks it is ok. But, it is very difficult to reach the second limit switch. It is behind the main air blower fan. How to reach it or I have to completely dis-assemble the while thing?
Thanks
 
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Old 11-13-15, 06:42 AM
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Update for my status. It is about LENNOX model G40UH-36A-070-02.
The error code is: Primary/secondary Limit switch open.
Both Rollout switches works fine.
Measured the primary limit switch, it is fine. Measured the second limit switch, it is fine.
But, the connection for the second limit switch connector to the circuit with primary limit switch is not stable. I will jump the connection and do one more test today. If it works, I may just use direct wire connections instead of plugs.
 
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Old 11-13-15, 05:34 PM
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Does your draft inducer motor run? On my York furnace, I got a pressure switch open error and even though the draft inducer motor was running, the fan wasn't spinning. Come to find out, there was a dead bird caught in the fan.

BTW, have you had look at the filter? If it's plugged up, that might be the cause because of insufficient air flow.
 
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Old 11-13-15, 05:40 PM
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Hello Crab,


Sounds like good troubleshooting on your part.


But Hongtau is getting a diagnostic code that his limit switch is open, not that the pressure switch you were dealing with isn't closing.


Different problem.
 
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Old 11-22-15, 10:01 AM
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G40UH-36A-070-02. The gas burning canít turn on

Hi, friends,
I still in the same trouble as before. The weather was warm and I am very busy recently. And, I have upstairs heater still function. So, I just put this thing aside for almost two weeks.
The problem is still still problem. I measured the wires of both limit switches and all in close circle position and nothing is wrong.
I twisted the wires once and the heater back to work again. This make me believe it is connection issue. So, I try to directly connect the wires (these blue wires) without the while plugs. And, the thing become more puzzling.
Finally, I skipped both switches and try to by pass both switch by direct connection both blue wires from the circle board.
The light on less than one minutes and both lights off afterward. Looks I can't by pass the limit switch connection.
Any suggestion for what I can try next?
Thanks
Hongtao
 
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Old 11-22-15, 04:18 PM
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Frankly, I suspect that you aren't testing the limit switches properly.

The diagnostic code is signaling that you have an issue there, and I suspect that is correct.


Personally, I set my meter to measure AC voltage and connect one test lead to furnace sheet metal as a ground.

Then I can use the other lead to check for 24 VAC at any point.

You should probably be able to measure 24 VAC where that circuit begins and ends and find 24 VAC at both points if everything is working properly.

First you want to find out whether you are losing that 24 VAC voltage SOMEWHERE in that circuit.



Then I'd walk the circuit of your limit switches, checking each side of each switch when the thermostat is calling for heat.

You should have 24 VAC on both sides of the primary limit and secondary limit.

You can check from point to point until you find a limit switch or connection that is failing to pass that voltage on.
 
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Old 11-22-15, 07:22 PM
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Please allow me ask a stupid question. Limit switch is just a kind of switch which is closed in normal situation. If I use a wire by pass the switch, the whole thing should work as normal (at least, it will start burn gas).
I need think more about this.
 
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Old 01-30-16, 04:02 PM
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I have same issue, have you figured anything out?
 
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Old 01-30-16, 05:30 PM
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You need to remove the blower door. On it should be a wiring diagram. You need to locate the limit loop and where it connects to the control board. Then you connect your leads AT THE CONTROL BOARD to those two terminals. Set your meter to over 25vac. The meter should show 0v when the limit loop is normal.

Verify that loop opens when you get a limit failure and then you'll know if it is or isn't a limit problem.
 
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