Carrier 59SC2B Error Code Code 33: Limit Circuit Fault

What Does Code Code 33 Mean?

Code 33 on the Carrier 59SC2B is a Limit Circuit Fault. It means a limit switch or flame rollout switch has opened (triggered) during operation. The control board responds by running the blower for 4 minutes or until the switch closes again, whichever is longer. This is the furnace's way of cooling down the heat exchanger to a safe temperature.

If the switch remains open for more than 3 minutes, the code escalates to Code 13 (Limit Circuit Lockout), and the furnace shuts down completely with a 3-hour auto-reset timer. If the switch closes within 3 minutes, Code 33 continues flashing until the blower shuts off, and the furnace may attempt another heating cycle.

The limit switch trips when the heat exchanger gets too hot, almost always because of restricted airflow. A dirty air filter is the single most common cause. Blocked supply registers, closed return vents, or a failing blower motor can also reduce airflow enough to overheat the heat exchanger. A flame rollout switch, on the other hand, detects flame escaping from the combustion chamber — this is a more serious condition and the rollout switch requires manual reset (it has a small button on the switch body).

If limit or rollout switch remains open longer than 3 minutes. The system will escalate to error code 13 .

Common Causes

How to Fix It: Check Air Filter and Vents

What You'll Need

• Replacement air filter (if needed)
• Flashlight

Steps

1. Turn off power at the breaker and shut off the gas supply valve Locate the circuit breaker for your furnace and flip it to OFF. Find the gas shut-off valve on the gas line leading to the furnace and turn it to the closed position. If you smell gas at any point, leave the house immediately and call your gas company from outside.
2. Check and replace the air filter Remove the air filter from the return air duct or furnace cabinet. Hold it up to a light source. If light does not pass through easily, the filter is too dirty and must be replaced. Insert a new filter of the correct size with the airflow direction arrow pointing toward the furnace.
3. Open all supply registers and return vents Walk through every room in your home and make sure all supply registers and return air grilles are fully open and unblocked. Move furniture, rugs, or curtains away from any vents. Check that no duct dampers have been accidentally closed.
4. Inspect the area around the furnace Make sure the area around the furnace is clear. The furnace needs adequate clearance for air circulation. Remove any items stored against or on top of the furnace.
5. Restore power and monitor Turn the gas supply back on, then turn on the breaker. Set the thermostat to call for heat. The furnace should start a heating cycle. Monitor it for 10-15 minutes to see if it completes the cycle without tripping the limit switch again.

When to Call a Professional

Contact a licensed HVAC technician if:

• The code returns after replacing the filter and opening all vents
• A flame rollout switch has tripped (this has a manual reset button and may indicate a cracked heat exchanger or blocked flue)
• The blower motor sounds weak, makes grinding noises, or does not spin up to full speed
• The furnace repeatedly short-cycles (runs for a few minutes then shuts off)
• You notice an unusual or burning smell coming from the furnace