Furnace blower fan cannot be auto run

Hi All,
I have a Lennox furnace (G26) with a problem now: the blower fan cannot be auto on or off, I should always put it to ON from the controller. If I put it to Auto, it wouldn't work.
Please help me figure out the problem.
Thanks,
Patrick
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On Monday, March 9, 2015 at 1:53:10 PM UTC-4, snipped-for-privacy@gmail.com wrote:

What happens when it's on auto? Furnace fires up, overheats, then shuts down? What age is this furnace? If it's an old furnace, they used a thermostatic switch that turns on the blower when the plenum gets warm enough. Newer furnaces for probably last two decades typically have the on/off function designed into the main control board. If you have the former type, then it's probably the thermostatic switch. If you have the latter, it's a bit strange, because the controller board also has to turn it on in response to the thermostat fan switch calling for it to be turned on when you do it manually. So, it would seem more likely that the fan just wouldn't go on at all, but it's possible a fault on the controller board could cause it.
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Thanks.
When turning it on Auto, the furnace can fire up, but the blower fan will never on. This causes furnace to be overheats then shuts down.
The furnace is about 8 years old.. where is the thermostatic switch located most likely?
Thanks,
Patrick
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On Monday, March 9, 2015 at 2:53:17 PM UTC-4, Patrick Feng wrote:

With the model number you should be able to find a wiring diagram online. Or there typically is one on the inside of one of the removable panels that give you access to the two furnace compartments. That would show how it's wired, if it even has a switch. If it's only 8 years old, it may not have a thermostat switch for the blower. Newer ones are just timed to go on X secs after the furnace fires up. If it has a switch, it's located somewhere near the top where it can measure the plenum temperature.
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On 3/9/2015 2:53 PM, Patrick Feng wrote:

to be overheats then shuts down.

Under the cover, front wall, near the burners.
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Stormin Mormon wrote:

Lennox G26 series may or may not have primary limit switch(Lennox call it) depending on model. They use several different control board.
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snipped-for-privacy@gmail.com wrote:

Your G26 is high efficiency furnace with at least 92% efficiency. When it fails(going down with over heat) you should read trouble code. Two LED indicators blinking. There must be inside the furnace cover panel, you should have a trouble code chart or in your owner's manual. When you say you keep the fan running from furnace side. Are you jumpering G and R terminal on the control board thermostat interface terminal strip? If so Did you try same from your thermostat side? Trouble-shooting requires logical steps. 3 things to check is in order is thermostat >> wiring from thermostat to furnace >> control board. Good luck. Also check you warranty some comes with 10 year warranty. Our furnace came with 10 year P&L warranty. 黃公
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On Monday, March 9, 2015 at 4:19:00 PM UTC-4, Tony Hwang wrote:

How would a fault in either the thermostat or the wiring to the thermostat result in the furnace starting, firing, the fan not coming on, furnace safety then shutting the furnace down? And similarly, how would a fault in either of those explain the fact that the fan works with the manual fan selection at the thermostat? Every furnace I've seen, if that problem description is accurate, the problem has to be at the furnace.
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On 3/9/2015 6:57 PM, trader_4 wrote:

95% of the heating and AC posts here start with "I replaced the thermostat and the heater still does not work" Its a tradition.
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Ed Pawlowski wrote:

I suggested to check three parts including control board. I did not define the problem. I wish I had a magic to point my finger right off the bat to the problem.
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Tony Hwang wrote:

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Tony Hwang wrote:

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Tony Hwang wrote:

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trader_4 wrote:

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trader_4 wrote:

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