Hot Topics: Furnace Switch Repair

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The Doityourself.com Forum features conversations between more than 250,000 experts and novices in over 120 categories—each week, we highlight a conversation that may help with your next project. This transcript has been lightly edited.

Original Post: Carrier 50M56U-751 High Temp Limit switch

the_dude - Thread Starter

I have a carrier single stage furnace that I replaced the control board in last year as I wouldn't fire up. It's worked fine until now. The error is gives is code 4 high temp limit switch. It tries to fire and the igniter glows but then shuts down. My question is I understand the high temp limit switch is a fan limit switch but is it an integral part of the control board on this unit? If so I'll need to replace the whole control board again.

Thanks guys!

the_dude - Thread Starter

Also the fan is stuck on and wont stop running.

roughneck77 - Member

It’s a separate part.

Is the burner firing or does it not make it past ignition trial?

the_dude - Thread Starter

Igniter heats up and then shuts off - the burners fire for a couple seconds and then cuts out.

roughneck77

Sounds like the limit is stuck open. You can test it with your meter.

the_dude - Thread Starter

Ok will do. I imagine they cost less than the control board. Maybe I should just replace it anyway (if it is stuck it would surely stick again if I managed to get it working).

the_dude - Thread Starter

How can i tell which one is the limit switch? From the pictures I've seen online a rollout switch looks very similar to a limit switch.

roughneck77 - Member

Limit is in the discharge airstream above the heat exchanger.

The furnace isn’t supposed to cycle on the limit so I’m sure you’ll find the cause after you get the limit issue taken care of.

the_dude - Thread Starter

Thanks, roughneck. do you mean i’ll find the cause of what stuck the limit switch open? i had this exact same problem last year and a guy who tested it told me it was the board and that the limit switch was fine. i replaced the board at that point and it worked fine for 10 months but its done it again. is it possible the limit switch is burning out my board?

roughneck77 - Member

No, it’s possible there’s a reason the limit tripped or was tripping. They are meant to trip during an unsafe condition, not during normal operating conditions.

the_dude - Thread Starter

Got it. I'll figure out limit switch and see what pops up next.