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First of all, when the unit locks out the circuit board should flash
an error code. Read this code before power is removed. There should be a sticker on the back of the panel door to the furnace with a wiring diagram and a list of the flash codes. example 2 flashes followed by 3 flashes = error code 23.
I bet the code will read flame detection fault. if it does you need to sand the metal rod that sticks into the flame of the furnace. this rod conducts electricity in the presence of a flame and is prone to oxidation which will keep it from reading flame correctly and shut the unit down intermittently until it locks itself out.
Thanks Joe, This unit has a Johnson Controls
G891TCA-8101 control module. It was a replacement for the original of a different type that failed long ago while the furnace was still under warranty. I've never seen any flashing codes and no hint of it having that capability in it's. It does have a flashing mode and 60 minute retry but it's not doing that, the green light is going out, which prescribes a power cycle to reset it. I neglected to mention that also last January when I changed the air filter, I inspected the spark plug and cleaned the flame sensor with steel wool. It didn't look bad, but the chats said that would be so even if it was bad, so I made sure to really polish it up good. That didn't seem to affect the intermittent problem which at the time was about every other week.
missing end of 3rd sentence above: ... no hint of having that capability in the on-line manufacturer description.More on the purge blower. I noticed the furnace circuit diagram shows line power into the controller, power passing out of controller to purge blower through a relay contact to the blower, then return line to neutral. So a simple circuit switched by the controller. Last night I tapped on the controller with my knuckle and the purge blower came on. No amount of tapping again or trying a heat gun (hair dryer) on the controller made it act up again.
So at this point I suspect the controller, but would hate to replace that expensive part and still have the problem.
After a full cycle so with no heat demand, the same conditions for the above random purge blower start, I was tapping around and purge blower started again, ran a minute or so and shut off. I let it sit a while and ran it through another cycle and tapped and poked around again for a while and purge blower cycled on and off again when not touching the controller, then let it sit and purge blower cycled again a few minutes later without me touching anything, waited for it to happen again but nothing happened. Next full cycle didn't do any of that.
Update Sat am: 1) I wanted to check the flame sensor per your original theory. I don't have the Lennox current to voltage transducer mentioned in their manual I got on-line. But looking at it, it doesn't seem to have any power source so I figured it must simply be a 1 Mohm resistor in series between the controller and sensor per their clip lead connection instructions, with the voltmeter across the resistor (1 uA * 1 Mohm = 1 volt to give the 1 uA per volt they mention) . I hooked that up (560K + 470K resistors, measured 1.14Mohm on my Harbor Freight digital multi-meter) and with flame measured 820 mV DC with same meter. During this test the furnace worked fine through its entire cycle. Specs say reading should be 1.2 to 2.2 vdc for the G891 (Johnson) controller. For kicks, did the test with just the 560K ohm resistor and measured 590 mV so that's about a 1 uA signal, getting close to the expected range of 1.2 to 2.2 uA. Since I'm not using the regulation test equipment, I'm not sure whether this means my flame sensor is in or out of spec. Since the furnace ignites and runs fine once started, unless the G891 controller tracks weak sensor voltage for a while and then delays locking out until after a heat cycle, I don't think it is the flame sensor.
2) Witnessed a couple more random purge blower (PB) on/off cycles during no heat demand period. Once today, PB was still on when main air circ fan stopped (when should have only run 30 sec after burner turned off (I think I've heard the click and felt the PB stop at this right time when everything seems to be working by the book; hard to positively hear this due to the main fan noise). PB ran about 5 minutes more and shut off by itself. Did not come back on this time, but I once witnessed it happen twice during one no heat demand period. This makes me suspect the G891 controller based on manual description that it is purely under the control of G891 and isn't supposed to operate then (unless G891 has some purge mode that I find no mention of where if it thinks the flame sensor is weak but not so bad as to be unreliable, it runs the blower; I think this is a stretch).
I have an appointment for 9 am Monday for Heating Service unless I can figure out the problem by then. Hope you can help.
Heating Repair service came by this morning and also thinks it's the G891 controller so I ordered one on line for much less than the heating company charges ($105 with S&H versus $344 + installation). The best I could get with the "W" wire disconnected was this evening. With it connected, the purge blower started randomly during the no call for heat cycle, I disconnected "W" and waited for more than 30 sec and purge blower continued to run. Cycled furnace power off and on and purge blower did not come back on. Didn't get a purge blower comes on with white wire off which would be the clincher. Thanks for your help. I'm accepting your posts. Thanks for the help.