What the Error Code Means
Error code 33 usually means the furnace sensed higher-than-normal heat and paused to protect itself.
The system is detecting a safety condition rather than pinpointing a single failed part.
Common Symptoms You May Notice
- The furnace starts but shuts off before warming the home.
- The blower may keep running while the burners stop.
- You see the 33 code flashing on the control panel or indicator light.
Most Common Causes (From Least to Most Serious)
- Restricted airflow: A dirty filter, blocked vents, or closed registers can reduce airflow and cause the furnace to overheat.
- Return air is limited: Furniture, rugs, or storage blocking return grilles can make it harder for air to circulate properly.
- System is running hotter than expected: In many cases the furnace is reacting to unusual heat conditions, which can happen during long run times or with low airflow.
Safe Things You Can Check Yourself
- Turn the thermostat off, wait a minute, and set it back to heat to see if the code clears.
- Replace or clean the furnace filter if it looks dusty or clogged.
- Make sure supply vents and return grilles are open and not blocked by furniture or rugs.
If at any point this feels unsafe or unclear, stop and contact a professional.
When to Stop and Call for Service
- The code returns quickly after a reset or during every heating cycle.
- You smell burning, see smoke, or the system shuts down repeatedly.
- The furnace will not produce heat even after basic airflow checks.
Typical Repair Cost Range
$150-$450 is typical for diagnosis and service to address furnace overheating or airflow issues, depending on the cause and parts involved.
Costs vary by region, model, and provider. Request an estimate before approving any repair.