The pilot light on our Lennox G40UH(X) Series furnace won't light. We went through the steps to restart the furance. The blower turns on for a few minutes but blows cold air and the pilot light never comes on. How can I fix this?

The LED lights are indicating that the primary or secondary limit switch is open (per the manual). The manual says that the limit must close within three minutes or the unit goes into a one hour watchguard. How do we correct this?

Check/replace air filter if clogged. Locate the limit devices, press/reset any with buttons. If the primary limit is stuck open, tapping the fiber plate with the butt of a screwdriver may cause it to reset.

Is it possible that one of the limit switches is bad? At first I thought that maybe we just needed to reset something but nothing seems to be working.

When we flipped what we thought was the limit switch it gave another error about the roll out switch being open. So what we thought was the limit switch was apparently the roll out. We can't find anything else that looks like it could be the limit switch.

I don't understand why they don't make user manuals that actually show real picutres of the system and where components are located.

Thank you so much for your help!

Roll-out switches are typically near the main burners, and resemble the previous pics.

After much googling I have finally determined where the primary limit switch is. I guess it is a flat metal plate with two wire attached to it. I saw someone post another question on another forum with the exact same problems we are experiencing and the responders suggested that the primary limit switch is stuck open and that tapping it might close it.

We tried the restart sequence again and tapped the primary limit switch during the three minute reset period and the burners FINALLY ignited. Our furnace is working again!

I just thought I would post this in case anybody else experiences a similar problem.

Wholy Cow... this works. We found this post, tapped on the primary limit switch and the furnace started up again. So simple. Glad we didn't have to pay to get the HVAC guy out here for an emergency visit!

Just a note... the primary limit switch on our G40UH is a red metal plate that is on vertical surface. It also has two wires coming out of it.

I had the same problem just now when I came back from X-mas with the in-laws. I needed a hot house and used your suggestions. It failed the first time, but then after tinkering with it, it worked for me. THANKS

The info about the primary/secondary switches was great. I actually unscrewed the red metal plate, pulled the plate out, and tapped/cleaned the dime sized (in diameter; about the thickness of 3 dimes) metal part when just the tapping did not work. I took apart the wires and rehooked them too for a clean/fresh connection. After cleaning the dime sized object I noticed 2 more dime sized objects with others wires leading to them as well. I unscrewed, tapped, and cleaned those as well. All the wires from these 2 similar looking dime sized objects were leading to the main gas switch that I was promted to turn to off before beginning based on the user manual.

Everything fired up nicely afterwards. The posting on the "red metal plate connected vertically" really helped me locate where the switch was. It's too bad the owner's manual does not go into more detail about this.

Thanks for the internet as well! It's nice to find out information and do it yourself!

I had the same problem, but I can't find it. Would any of you can work me through the procedure you did?

We are cold to death as it is only blows but not burning.

If you can call me to help out would be greatly appreciated.

Please do not post phone numbers. Thank you

John

I had the same problem just now when I came back from X-mas with the in-laws. I needed a hot house and used your suggestions. It failed the first time, but then after tinkering with it, it worked for me. THANKS

The info about the primary/secondary switches was great. I actually unscrewed the red metal plate, pulled the plate out, and tapped/cleaned the dime sized (in diameter; about the thickness of 3 dimes) metal part when just the tapping did not work. I took apart the wires and rehooked them too for a clean/fresh connection. After cleaning the dime sized object I noticed 2 more dime sized objects with others wires leading to them as well. I unscrewed, tapped, and cleaned those as well. All the wires from these 2 similar looking dime sized objects were leading to the main gas switch that I was promted to turn to off before beginning based on the user manual.

Everything fired up nicely afterwards. The posting on the "red metal plate connected vertically" really helped me locate where the switch was. It's too bad the owner's manual does not go into more detail about this.

Thanks for the internet as well! It's nice to find out information and do it yourself!

I just got my furnace up and running with this information. Thanks!

KC13 you are the PRO. THANKS>>>>>>THANKS>>>>>THANKS... soooooooooooooo much

Limit swich is the "red metal plate connected vertically" and has two wires (purple and blue) going to one terminal, and one wire (blue) going to the other. The two blue wires are supposed to create a closed loop through the switch, so with an ohm meter you should have continuity between the two terminals. Both blue leads originate at the circuit board, and the purple lead goes to the pressure switch (which is attached to the induced draft blower). I did just as the others states, removed the switch, disconnected leads, tapped everything on the switch with the butt of my screwdriver, and then reinstalled. Worked like a charm. Please note: the pictures at the beginning of this post are of two rollout switches, and the last smaller image of the square plate with a sensor extended from it is the actual "primary limit swich."

on a cold night guess what my wife noticed the heater stopped blowing hot air.So what u going to do about it well like a man I Took a look at it. Sure enough had a blowing fan running with no gas burning. She was right again. Well got on the web and read another website that told me to clean the flame sensor for what reason I don't really know but did it. Recycled the system again and the same problem. The 1st led light slow flash and 2nd led light on continuous so back inside I went trying to get an answer so lucky I found this post the exact solution to my problem. Took the reddish colored board off and unplugged one of the wires so it could be reached better. Tap Tap Tap and a little more tap just to make sure and tap again. Well installed it back and reset the system again. Turn the power back on and made sure both led lights where flashing witch means normal then turn the heater on ran outside to see what was happening well both led lights where still flashing and the heater comeback to life thanks for the solution for g40uh x combustion primary or secondary switch open.

How do you reset the g40uh

What are the steps for restarting a
g4ouh furnace?

Factory service manual for G40UH(X)

http://0190acd.netsolhost.com/manuals/G40UH_Series.pdf

https://www.hvacpartsshop.com https://www.hvacpartsshop.com/83l90--limitcontrolprimary.aspx
https://www.hvacpartsshop.com/17w70--ignitioncontrol24vcircuitboard.aspx

I also saw this forum, I am a renter. And after my landlord/brother... who is a plumber and heating guy... looked at the furnace, he could not find the problem.. I said the heater indicates that it was the primary and secondary switch. He called another heater guy, who came out and said it was the circuit board! A week later the board came in, my brother... I mean landlord, wasn't going to be home for awhile, so I decided to look it up online, I found this forum and when I tapped on the primary it worked... I was so pissed!! I could have done that a week ago! Thanks everyone for the help!

I wanted to let you know that I had the same issue... and tapping on the red plate worked like a charm. Amazing that it was that simple.

Thank it work!

Yup, a little tap with the butt of a screw driver and it works like a champ!

This works. That save some money. Make sure that your batteries are new in your zone controlleralso.

Can someone please elaborate on where the fuse is found in this model?

My unit is getting no power at all, and for the life of me, I cannot find the stupid fuse to try and replace.

Same issue, same solution. Weird it was such a simple fix. I didn't even unscrew the little red plate, I just tapped it with a screwdriver handle. I shut off the power to the furnace and turned it back on (we have a light switch style switch installed on the ceiling by it that I didn't even know existed until now). The lights went from slow flash red- solid green before shut off to flashing simultaneously after shut off and next thing I know I see flame. Good stuff! The manual was freaking useless but this thread worked like a charm. I didn't even know what Watchguard was or what to do about it. Thanks random website that Google steered me to!

The LED lights are indicating that the primary or secondary limit switch is open (per the manual). The manual says that the limit must close within three minutes or the unit goes into a one hour watchguard. How do we correct this?

The answer by bmwsRus was the best. On mine the plate is black, the switch is at the end of a 4" rectangular plastic post. But the color of the wires are the same, I disconnected the wires, unscrewed the unit, took it out, taped the end of the switch, cleaned the rest, put it all back together and it worked great. Thanks everyone. Kudos also to Lennox, the way the screws were set slightly off center at the top made it so much easier to put back correctly.