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?

this sound like you gas valve is stuck

If its a gas furnace it could be the heat sensor on the pilot flame unit. When the sensor is failing it stops telling the control that the pilot is lit properly and the furnace goes thru another start up sequence.
You can try to scrape any flash and dirt off of the heat sensor rod to clean it or remove it and clean or replace the sensor or the pilot assembly. There are plenty or furnace parts on Amazon. The next most likely thing is the relay board is ready to fail that part is ~$80 but you need to get the part number right and be experienced with wiring.
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
Be Safe :)

you will need to verify voltage from the board to the inducer. If you have the installtion manual there is a flow chart for the sequence of operation of the furnace.
The basic sequence is:
1. thermostat calls for heat
2. inducer motor starts and runs for a pre-set amount of time
3. the pressure switch closes and allows for ignition trial
4. ignition is proved back to the board allowing the system to operate.

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

Hi check out the flame senser is clean also jump ou tthe pressure switch see if it lights when you do this , igntion modul is just a box with some push on wires just get the right one the info is all on the module

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. ???

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).

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

Turn the fan to the off position on the tstat. You colud have a ilimit switch or a relay on the control board if it has one.

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

change the filter.