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Old 02-20-2012, 08:49 PM   #1
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Lennox G20E Gas Furnace Down; What next?


Following excellent and detailed advice on this forum, I have cleaned the flame sensor with emery paper but the furnace has refused to work. The furnace was down twice before this year but the flame-sensor cleanup restored the operation the last two times. What should I try next other than (1) replace the flame sensor (most likely the entire sensor+ignitor+pilot burner) and (2) call a pro if that fails (let me know of you know of a good pro near Lisle/Naperville, IL area).

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Old 02-20-2012, 09:00 PM   #2
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Lennox G20E Gas Furnace Down; What next?


How old is the furnace?

Try looking at the control board without disconnection power and see if there is a flashing light. Count the flashes and then look for the flash code chart on the furnace.

You can also post brand and model with the flash code.

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Old 02-20-2012, 09:07 PM   #3
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Lennox G20E Gas Furnace Down; What next?


Quote:
Originally Posted by Plumber101 View Post
How old is the furnace?

Try looking at the control board without disconnection power and see if there is a flashing light. Count the flashes and then look for the flash code chart on the furnace.

You can also post brand and model with the flash code.
Furnace was installed in 1994. The Control Board has no flashing LED. It has Robert Shaw 7200 Gas Valve and Lennox SP 745 Ignition Control Module. I did a current test on the flame sensor and I am getting about .1 uAmp (on 750V; not sure how to set my multi-meter to 24V). Anything can be bad but I would like to make sure that a part is definitely bad before replacing. Any of these parts can by >$100 or more each.
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Old 02-20-2012, 09:21 PM   #4
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Lennox G20E Gas Furnace Down; What next?


Meter must be on micro amps setting.

You control board is bad.
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Old 02-20-2012, 09:27 PM   #5
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Lennox G20E Gas Furnace Down; What next?


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Originally Posted by hvac5646 View Post
Meter must be on micro amps setting.

You control board is bad.
My multi-meter was on the microAmp setting. It only read 0.1 microAmp (but it is being measured at 750V).

By control board, I guess you mean SP 745 (and not the BCC2 control board by the fan). Correct?

Just curious how you so quickly narrowed down the problem to SP 745. Is it a common problem?
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Old 02-20-2012, 10:48 PM   #6
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Lennox G20E Gas Furnace Down; What next?


So you are measuring .1 microamps after the pilot lights, or after the main burner is on. How far through the heating sequence does the furnace make it? Can you hear the spark ignitor try and light the pilot? Does the damper door open all the way infront of the burners?
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Old 02-21-2012, 09:50 AM   #7
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Lennox G20E Gas Furnace Down; What next?


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So you are measuring .1 microamps after the pilot lights, or after the main burner is on. How far through the heating sequence does the furnace make it? Can you hear the spark ignitor try and light the pilot? Does the damper door open all the way infront of the burners?
Since the pilot is not getting lit, I took out the flame sensor assembly and warmed it over a stove. I got 0.1 micro AMP (across 750V, the multimeter says).

Right now, SP745 seems to be in a lock-out and won't even try to start the pilot. Grounding checks out. I have jumped W and R to kick-start the furnace but nothing is happening anymore. No action from ignitor. No spark. Nothing.

If control box won't even respond, I have no choice but to swap it out and hope for the best.

I noticed that SP745 has been superseded by 30W3301 ($130 online with a new flame sensor) or by ICM293 ($120 without flame sensor). Which one would you suggest?

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Old 02-21-2012, 09:56 AM   #8
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Lennox G20E Gas Furnace Down; What next?


Does the damper door in front of the burners open? This has to happen before there will be a spark to light the pilot light from the contol module. If the damper motor is burnt out, or the small micro switch that the damper motor makes when it opens doesn't work you won't get to your trial for ignition.
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Old 02-21-2012, 10:03 AM   #9
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Lennox G20E Gas Furnace Down; What next?


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Originally Posted by HVACDave View Post
Does the damper door in front of the burners open? This has to happen before there will be a spark to light the pilot light from the contol module. If the damper motor is burnt out, or the small micro switch that the damper motor makes when it opens doesn't work you won't get to your trial for ignition.
Damper door opens just fine. I haven't tried to jumper the switch but I think it's working since the SP745 is indeed getting 28V (it should only be 24 but not sure if 28V will hurt anything) at the input terminal (TH).
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Old 02-21-2012, 10:07 AM   #10
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Lennox G20E Gas Furnace Down; What next?


So now you are to the ignition control module. If there is no trial for ignition of the pilot, those modules were prone to failure. Is it the grey Robertshaw module, or the blue honeywell?
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Old 02-21-2012, 10:11 AM   #11
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Lennox G20E Gas Furnace Down; What next?


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Originally Posted by HVACDave View Post
So now you are to the ignition control module. If there is no trial for ignition of the pilot, those modules were prone to failure. Is it the grey Robertshaw module, or the blue honeywell?
Grey Model (but says Lennox SP 745 on it). I guess I am lucky that it lasted as long as it did (16 years). I will go ahead and order it. Not sure if I should get Lennox 30W33 w/new flame sensor or ICM293 w/o flame sensor. Any advice?
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Old 02-21-2012, 10:28 AM   #12
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Lennox G20E Gas Furnace Down; What next?


If the new flame sensor is the upgrade it should fit, but I haven't tried that part before. Replacing most of those older models now with high efficients, but installed litteraly hundreds of them back 15-20 years ago.
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Old 02-21-2012, 11:10 AM   #13
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Lennox G20E Gas Furnace Down; What next?


I was just wondering what kind of meter you have. All the meters I have used showed only the u-amps and not the voltage.

All if not most signals use a carrier voltage of 120v. And that was generated by the control module.

How do u get 750 v off a stove flame?
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Old 02-21-2012, 11:20 AM   #14
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Lennox G20E Gas Furnace Down; What next?


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Originally Posted by hvac5646 View Post
I was just wondering what kind of meter you have. All the meters I have used showed only the u-amps and not the voltage.

All if not most signals use a carrier voltage of 120v. And that was generated by the control module.

How do u get 750 v off a stove flame?
I have an old Honeywell yellow big multi-meter. I may be reading it wrong. On the micro AMP setting, it gave me a current of 0.1. The terminals have a signage of 750V so I may have mistakenly reported it.

As I said, since even pilot is not lighting right now, I had no choice but to warm up the flame sensor probe on the stove and connect my multi-meter in series (I grounded the other end).

However, since the pilot is not even getting lit, the problem lies upstream - possibly in the ignition control module. Don't you agree?
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Old 02-21-2012, 11:50 AM   #15
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Lennox G20E Gas Furnace Down; What next?


If you are getting 24-28 volts at the input to the module then it should produce a spark unless the spark wire is shorted to ground and that can happen as it dries up and goes thru sharp metal. Take the spark wire off and hook an alligator clip and wire to the terminal and hold it 1/2 inch away from the metal box and wear gloves and see if it sparks. 99% sure it is the module and definitely go with the Honeywell 30w33 from Cozyparts.com as it is the Lennox part and has great install info. also has new built in diagnostic features which is better than any generic unit.

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