Sunday, February 21, 2016

Fixing Carrier 90 Gas Furnace Limit Switch (#13 and #33 Error)

Fixing the #13 and #33 error on a 90% efficiency carrier furnace with variable speed blower motor

Difficulty Level (Easy, Medium, Hard, Insane):

Two nights ago before going to bed I noticed that the house seemed colder than normal so I checked out the thermostat and saw that there was a "System Malfunction" notice. I decided to go down to the furnace room and have a look at it.

I had a slight suspicion as to what the issue could be and just like I expected I saw the #13 error. After consulting the error code section in the manual that came with the furnace it specified a "Limit circuit lockout" error.

The reason I was expecting this error was that two days ago I got the #33 error (Limit circuit fault).

Basically what that means is that one of the thermal cut-off (TCO) switches would trip and throw the #33 error, but this time it seems like the TCO/Limit switch failed and got stuck in the open position.

As it was getting late, I decided to turn off the furnace main switch and see if it would reset over night. Unfortunately, at 6:20am when the house should be warming up it wasn't so I knew I'd have to do some more trouble shooting and then fix the issue.

I hauled out the volt meter and confirmed that the limit switch was in fact showing no continuity and had failed in the open position.

I disconnected the two wires and temporarily by-passed the limit switch and spent the next hour sitting beside the furnace while the house was heating up to a nice temperature. Of course, I couldn't leave the furnace on without the safety feature of the limit switch but with my stay at home wife and three small kids I wanted to make sure the house was at least not cold.

After some more research I concluded that the two most likely causes for the error were:

1. Stuck high-heat gas valve
2. Broken limit switch

I suspected #2 because admittedly, I had not been very diligent in cleaning out my furnace air filter, which I supposed caused a reduced air flow through the heat exchanger and consequently a reduced transfer of heat away from the furnace which resulted in the tripping of the limit switch.

After sourcing a part through one of our local furnace repair shop I loosened the two screws, pulled out the old limit switch, put in the new one and everything was back to normal.

Luckily, it was just a $30 part and since I did my own labor it was a fairly cheap problem. However, if I had to get a furnace specialist to come in, trouble shoot the issue, order the replacement part and come back to install it I would have looked at a bill of at least $300 bucks.


Part of the trouble shooting manual showing the #13 and #33 errors
The limit switch
Old limit switch stuck in the open position
Manual override of the limit switch - NOT RECOMMENDED
Limit switch model
Carrier furnace type
The replacement limit switch
Brand new limit switch
Close-up of the limit switch
While I was at it I also cleaned the stainless steel probe
Temperature right by the limit switch. Clearly after cleaning the air filter it was within proper limits

Socket set
Screw driver
Volt meter

1 limit switch




House is nice and warm and no more errors


Unknown said...

On my Carrier Boost 90, showing faults 34 and 14, I cleaned the flame sensor ss rod and the igniter, checked the outside supply and return pipes for obstructions, but the gas valve still does not seem to open. Next I'll go for the limit switch that it is easy to do, other than that I am lost. What should I check next? Thanks.

Chris Eigenheer said...

there are some vacuum switches towards the right side of the main box that contains the burners. you might want to check those out. also, if you remove the cover with the round looking glass (temporarily only) does the gas ignite? You might have an air supply issue. good luck with your furnace.

Milan said...

Hello, my furnace runs non-stop and does not follow what i do with Ecobee thermostat suddenly over night. and it gets stuck with code 34 and then 14 after resetting it multiple times. still inconsistent - sometimes it starts and keeps on running and sometimes it gets stuck at 34 and 14 code. plese let me knwo if any suggestions. thanks !

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