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Gaztech, HVAC Technician
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Experience:  Owner at Gaztech
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I have a 20ish year old Lennox G26q3/4 100-1 heater. When it

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I have a 20ish year old Lennox G26q3/4 100-1 heater. When it starts, it works fine for the first few minutes, meaning it that it fires and the gas runs and heats like normal. Then after a few minutes, the gas shuts off. When the gas goes to turn on again the pilot turns on but then the gas seems to come on for a second but then immediately shuts off. It tries a few times and then just blows cold air forever.

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Customer: replied 11 months ago.
I am trying to send a video of the failure
Customer: replied 11 months ago.
Just answer won't allow me to send a video because of the file type. Is there any other way to send you a video?

Here is a link that shows the flash code you might be getting after furnace locks out.

You will find the blinking lights through a round sight glass at the bottom furnace panel

Customer: replied 11 months ago.
I haven't been able to see any blinking lights

take bottom panel off locate control board, there will be 2 led lights just above where the t-stat wire are tied in, make sure nothing is blocking your view when you look through sight glass

Customer: replied 11 months ago.
Ok. I'll try that.

refer to page 11 of link for diagnostic codes

Customer: replied 11 months ago.
My control board doesn't appear to have LEDs. At least not anywhere that I see
Customer: replied 11 months ago.
It also doesn't match the drawing in the book

does it look like fige 10 on page 11

Customer: replied 11 months ago.

OK that is the first generation G26,

stand by

go through page 14 and 15, your led is on ignition control

Then page 16 shows flame sensor access and maintenance

Customer: replied 11 months ago.
Ok. I am trying to go through that. Can we resume in the morning. This is going to take some time.
Customer: replied 11 months ago.
My ignition control looks like a Honeywell brand, not the Johnson controls that the book says. And I can't find an led on that either.

I have to go out to work tomorrow but try this net step if first one fails,

The other possible issue could be item 15 on page 17, along with figure 23

that is not the easiest furnace to do repairs on but with this info you might get it depending of your skill level

Good luck

Customer: replied 11 months ago.
Ok. Thanks a lot. I'll let you know how it goes. I appreciate your help.

Best of luck

Customer: replied 11 months ago.
I'll definitely try item 15. That is not too difficult. Thanks a bunch. I'll get back with you.


Did you fix the problem?

Customer: replied 11 months ago.
No I didn't. I found that a Honeywell ignition control was added 15 years ago and it has no diagnostic LEDs that I can find.

have you got around to clean the flame sensor?

Customer: replied 11 months ago.
I did that. No change

I am thinking that the ignition control where the flame sensor hooks in has gone faulty and they often do.

Customer: replied 11 months ago.
Ok to fix that should o replace the whole sensor or what would you recommend?

Considering the age of unit, it would be a good idea, I happen to have the exact same one and all controls were replaced 3 years ago.

Last thing are you able to view pilot flame at startup, it needs to be blue and long enough to heat the flame sensor, and even after flames are established, small pilot flame can get sucked out off position or completely

Customer: replied 11 months ago.
pilot is on. It lights the gas the first time then when it cycles off and back on again, it lights the gas but a mini explosion happens after about 3 seconds and the whole thing shuts off and then tries again about a minute later until I turn it off.

I am on phone with customer, will get back to you shortly

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