How To Test and Bypass The Upper Limit Switch On Your Furnace

If you have the 4 blinking red lights or know your switch is bad Ill show
you how to bypass it.

[capecodder]

Thank you so much!!!
My furnace stopped working over a week ago and I already bought an ignitor and a sensor and those did not work and I've been watching a lot of videos and finally decided to look up what 4 blinks means for the error code and found your video after getting an answer that it was the "upper limit switch".
Thank you very much for your help!!!
It's frustrating trying to troubleshoot this type of problem when you are not use to working on furnaces and you have to learn how they operate and what all the parts are for.

[randalleicher]

I appreciate the video. This poor father is one step closer to a working furnace this winter.

[LMFAOTVNETWORK]

OMG THANK U THIS SWITCH JUST WENT OUT AND ITS FREEZING HERE IN N.J. THIS GUY IS A LIFE SAVER. THANKS AGAIN...

[AlexTechWiz]

My furnace was going ON and OFF constantly, I found that the problem was the Limit Switch, I had this idea of bypassing it, but I was not sure about that, after watching this video, I did that, and bingo! No more ON / OFF. I just ordered it, will get replaced as soon as it arrives. Thanks a lot for the tip!

[JamesChen]

Thank you very much for this info. You are right my upper limit switch became faulty. Your tip worked exactly as you said. Thank you again.

[cliffnulman]

Thanks a million ! This got me back up and running in minutes.

[flashy]

Bypassing the switch can melt your whole furnace and blow up your house if the heat inside gets to high. “YOU REALLY HAVE TO MONITOR THE HEAT WITH THAT THERMOMETER AND DON’T FORGET ABOUT IT AND DON’T GO TO SLEEP WITH THE FURNACE RUNNING UNLESS SOMEONE IS THERE MONITORING IT CONSTANTLY. That switch is there so you don’t have to monitor it. It’s like a security guard at a gate, it protects you from certain harm. If you take that security guard away, your gate is vulnerable to the elements.

[jeremydechant]

Thank you so much. You got me the info that got me fixed.

[dantrello]

Thank you sir! You just saved me me $300.

[PaulIhn]

Great information - took me straight to the root cause Next step order limit switch I can't thank you enough...

[cowswirl]

At 2:44, there is text on the video that says "Make sure the upper limit switch is screwed in place. do not remove it if furnace is running." I'm confused. At that point in the video, the switch has been taken off the "wall", right? How do you screw the switch in place when you have a jumper cable (the wire with the black shield) on it?

[DJMovit]

I called a friend that works for an HVAC company. They didn't have the switch. They'd have it Monday when the local supply company opens.
I ordered one from Amazon and supposedly will have it tomorrow, if not Monday.
Keeping my thermostat at 68 degrees is helping pipes not freeze. It's currently 5 degrees Fahrenheit outside and going to drop even more.

[jdgk]

I have a 30 year old Carrier furnace which still had the original control board (HK42FZ011). Well, the blower stopped coming on so I ordered a newer, less expensive control board (ICM282B). I had no problem installing the the board. I switched on the system and it was running like a dream...until about 10 mins later when I started getting CODE33. I tried to jumper the limit switch as you instructed but I blew the fuse on the control board. Is it possible that it was because part of the metal connector on the jumper was touching the metal on the furnace? I still don't know why i never got this kind of error on the old board but now, if close up the furnace by putting all the doors back on, it will CODE33 in a few mins. However, if I leave the blower door off, it runs without issue. My guess is that, with the door closed, the blower is not pulling enough air into the system and is causing the limit switch to trip. Or, is it possible that the air pressor sensor is bad. I am about to give up and buy a new furnace.

[epryor]

Will the furnace shut off when it hits the thermostat temp set ?

[jimbarron]

Finding the root cause is safer. Likely an air flow issue. The high limit is very much a safeguard to problems with serious consequences and not to be trifled with.

[dougofthenorth]

I was having problems with the limit switch short cycling the furnace. I was skeptical to bypass it, for safety reasons, and decided to look a little deeper. I found that my heat exchanger was cracked, which was the root cause of the upper limit switch opening. I’ve deactivated the furnace as I don’t want to risk getting carbon monoxide poisoning. Just wanted to point out that bypassing safety switches should only be done with acceptance of risks.

[JonathanFlores-zuhn]

What if you jump the switch but the furnace still won't start, what else could cause it to show this code?

[kumaransivan]

I tried your approach and got it working! How long I can go on with this approach? I have monitoring setup and checking the temperatures every 30 minutes. I have folks coming here to replace the furnace and A/c in 3 days. Can I go on like this for 2-3 days? Checking the temperature at the vents for 120-degree is it good enough? thanks

[cowswirl]

where do i get a shielded cable? Does it need to be a certain type? Can I use any spare sheathed cable lying around the house?

[hotrod-machines]

I have a question I hope you can help with, I have a Trane xc95m that is randomly throwing a high limit error. I tested the switch and it appears to be fine, pulled the switch and it looks clean and new. Ironically when I used a jumper to bypass the high limit switch I ended up with a PS1 closed error 😞 I've ran the furnace with no filter in it, and the main door off but I still get a random high limit error. As you mentioned its usually earlier in the morning that I notice the furnace suddenly starts blowing very, very hard. If I go check on the furnace the high limit has been tripped, gas is off and the blower is running on high for a few seconds.

I'm not sure where I would go from here? I even took the furnace door off and opened all the heat vents in the house. After a day like that I was getting the same issue. I took the high limit switch out, and with my laser thermometer the outside of the burner box was 47.2C - 117 and if I shot the laser into the high limit hole opening it was 95C - 203F