Carrier Furnace Code 33 - Limit Switch Fault / Overheating

What Caused This Error?

The control system reports 33 when the monitored condition described by the error code is outside the expected operating range. In this case, the most likely trigger is extremely dirty home air filter restricting airflow. Other common contributors include blocked return air grilles or closed supply registers and failed primary limit switch.

Step-by-Step Diagnostic Check

1. Replace dirty air filter with a fresh unit immediately.
2. Ensure all supply registers and return vents across the home are open and unobstructed.
3. Disconnect electrical power before opening panels. Inspect blower fan wheel for dust buildup and test capacitor.
4. Disconnect electrical power before opening panels. Test primary limit switch continuity with a multimeter; replace switch if open when cool.