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


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