Lennox furnace wont ignite

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  #1  
Old 04-24-20, 08:43 PM
J
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Lennox furnace wont ignite

Model number - ML180UH070P36A-07

air filter brand new, fresh batteries in thermostat, gas is open from main.

thermostat calls for heat
induction motor comes on
hsi gets red hot
”click” happens
no ignition, retries 4 more times and goes into watchguard mode (alternating slow flashes) - burners failed to ignite
this primarily happens at night i dont know if that helps.

i have cleaned flame sensor, cleaned hsi (without touching it) all wires connected, no burnouts on control board.

any help would be appreciated thank you in advance
 
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  #2  
Old 04-25-20, 07:43 PM
R
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Check the voltage to the gas valve. If you have coil voltage check gas pressure.
 
  #3  
Old 04-25-20, 09:14 PM
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Welcome to the forums.

The HSI is extremely fragile and should never be touched.
I know.... there are many videos telling you to sand the igniter. Wrong.
Usually if there is an igniter problem..... the gas will sometimes light or will light with a boom.
If that igniter doesn't look brilliant white when heating.... replace it.

Roughneck mentioned the gas valve. That's what you need to check next. There should be 24vAC on the two wires for approx 7 seconds before the igniter goes out. If the gas valve has an off/on switch on it...... operate that a few times. Remove the wires from the valve and put them back on. Disconnect the large 12 pin plug on the main board several times. It's very possible to have a corroded contact.
 
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Old 04-26-20, 07:35 AM
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I have checked voltage and it has voltage at gas valve i cant remember off hand what it was and will check again. The hsi is definitely glowing red not white when you hear that click. I can replace it it’s relatively inexpensive just hoping its not gas valve issue
 
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Old 04-26-20, 03:16 PM
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The 24vAC at the gas valve will only be there when the valve should be open.
In other words.... if you are measuring 24v on the valve at all times.... you aren't measuring correctly.
 
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Old 04-26-20, 07:00 PM
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Understood, ive been watching a million youtube videos on it i get what youre laying diwn
 
  #7  
Old 04-28-20, 05:48 PM
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Straight from the service manual - do the testing shown in the picture below to make sure the igniter is still good and board is applying appropriate voltage to it.

The ignitor resistance may be too high, preventing it from warming up completely.

It should be bright orange to white when fully warmed up - not red.

 
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  #8  
Old 04-29-20, 04:39 AM
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Replace the Gas valve

I replace at least a couple every heating season. Often enough that that GV is truck stock.
 
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