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Lennox G60UHV – Ignition but immediately shuts down – alternating slow flash dia

Lennox G60UHV – Ignition but immediately shuts down – alternating slow flash dia


  #1  
Old 11-15-19, 01:07 PM
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Lennox G60UHV – Ignition but immediately shuts down – alternating slow flash dia

Hi – hope someone can help.

1) Thermostat calls for heat
2) Combustion Air Inducer turns on
3) delay
4) ignitor lights
5) gas valve opens and flame ignites
6) flame stays light for a few seconds, then gas valve closes (flame goes out)
7) Sequence repeats a total of 5 times
8) Furnace goes into Watchguard mode with alternating slow flash
“Watchguard −− burners failed to ignite; OR limit open more than 3 minutes; OR lost flame sense 5 times in one heating cycle; OR prove switch opened 5 times in one heating cycle.”

Steps I’ve taken:
1) Cleaned flame sensor & burner jet surface
2) Replaced flame sensor

Problem still existing

I’ve been told by a tech over the phone that the only other possibility is the furnace control board and he suggested replacing that.

Is there anything else to check? Would a tech coming out likely do any other tests to find any other possible problems or am I just wasting money having someone come out?

I can get a new board at dealer cost or have found used ones on ebay for less than $100. Any suggestions on whether trying a used board makes sense?

Thanks for any advice you can offer!
 
  #2  
Old 11-15-19, 09:38 PM
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Welcome to the forums.

A tech would not guess. He would connect a meter that could read microamps in series with the flame sense wire. You need to see what the operating current is. Many things can cause a problem with the flame sense circuitry.

The rod must be fully engulfed in the flame. All screws on the burner and control board should be checked for tightness. The metal chassis is one half of the flame sense circuit. Any rust or corrosion can cause problems.
 
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Old 11-18-19, 12:29 PM
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Thanks Pete. I should have mentioned that I tried connecting a meter in series with the flame sensor and it caused the system to not get to the ignition stage. It stops at the Combustion Air Inducer stage and doesn't continue to the combustion stage. Not sure why this would be the case and maybe I'm doing something wrong with the setting on the meter but I have it set to the 200u DC amp setting. Also, when I fully disconnect the flame sensor it does the same thing so apparently having the flame sensor connected is required to get past the Combustion Air Inducer stage.

The rod is definitely engulfed in the flame as I can see it through the little window and it didn't change position from when it worked and when it didn't.

I'm not sure what you mean by "The metal chassis is one half of the flame sense circuit" but I cleaned the surface of the jet right where the flame sensor sits in front of with steel wool so that area should be clean and able to conduct current.

Any thoughts on the failure to get the system past Combustion Air Inducer stage.when the meter is connected in series with the flame sensor or when the flame sensor is completely disconnected? Is that normal?
 
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Old 11-18-19, 02:42 PM
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so apparently having the flame sensor connected is required to get past the Combustion Air Inducer stage.
Not if it's a single wire flame sensor. That sensor is only used once the flame is established.
Is there a possible short on the flame sensor line ?
Disconnect the wire and check it with an ohmmeter. There should not be any continuity measured.

The circuit goes from the module thru the flame sensor thru the flame to ground. The flame creates a rectified DC. If the burners or the control board are not solidly grounded..... the sensor loop is open.
 
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Old 11-29-19, 06:05 PM
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Figured out the problem. Bad flame sensor wire. Took 3 techs to finally figure it out. Wire tested as having continuity but wasn't working properly. This is a good lesson to just go through each component and even jump out the wires if possible even if they test good on continuity.

Thanks for your help!
 
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