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I have a Lennox Conservator G8E furnace with Johnson Y79ABD-1 ignition control. More and more often now, when the thermostat calls for heat the ignition seems to go through it's spark/sense sequence and then shut down. Then of course the heat never comes on. Other times it operates normally. Do you have any explanation for this erratic behavior?

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  • bigblu Dec 03, 2008

    Thanks Joe. I received the same advice from HVACmechanic.com. You were both right. So far so good.

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You need to check the flame sensor and make sure it is putting out 4 to 6 microamps from the wire to ground

Posted on Nov 30, 2008

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Our Lennox Conservator G8E furnace goes through several ON-OFF cycles (we counted up to 10 clicks) before it stabilizes and stays ON. Once it's ON it works fine. - Any suggestions?


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WARNING-DANGER B4 you do anything to your furnace Turn Off The POWER first! If there is no plug or switch near by, shut of the circuit breaker! If you don't know what any of this means then you are likely not a good choice to mess with this device
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Verify 120VAC to the inducer motor upon call for heat from stat, if you have 120 at the leads that connect to the inducer and it does not start , the motor is faulty. If the inducer starts use a voltmeter across the pressure switch terminals, a voltage reading of 0 VAC will verify contact closure and ignition should occur.

The pulse furnace was under a factory recall due to breaches in the heat exchanger, unfortunately for you, the Lennox Corp. has ended the program that allowed for an allowance / furnace for replacement. To test the heat exchanger a special kit is required from lennox that will verify integrity of the heat exchanger. (If the heat exchanger seal has been comrimised the pressure switch will not prove and the igniton sequence will be interupted and cause the system to go into "lock-out" mode).I know this is probably not what you wanted to hear. Hope this helps, please list as "very helpful" if it did

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Lennox merit series furnace is not kicking on until house is 8 to 10 degrees colder then set thermostat


I have the same issue with a G21Q4-5-100-1 model lennox. House drops 8 - 10 degrees. I have 3 thermostats and all react the same way. I was told by service tech that the Ignition Coontrol Module is bad and acting intermittanely. ???

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Lennox diplomat furnace will not always ignite. I have cleaned the heat sensor and ignitor. Could ignitor be going?


Maybe, look for bad wiring connections and retighten the ground wires from circuit board to cabinet. Take a can of air and clean the burner tubes being careful not to blow on the igniter. Are ther any error codes blinking when it does not light? Read on for some more tips.

On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

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Is this on a Lennox gas furnace? If so you may be able to get one from lennox. Imay have one at work, but I won't be back there until Monday. bryan@reedwrightheating.com

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Blower motor wont shut off


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1 Answer

Furnace will not heat.


Most of the Lennox G-16s that I have repaired were having problems with the gray Penn-Johnson Piggyback igniton control module.
This is best repaired by replacing the spark module. Sometimes it may be neccesary to replace the gas valve also. Lennox updated to a ROBERTSHAW ignition system. Honeywell can also be used.
Also check that there is no water in the pressure switch or tube.
Hope this helps.
Matt
machtcl1@verizon.net

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