Carrier 59SC5A Error Code 33: Limit Circuit Fault

What Does Code 33 Mean?

Code 33 on the Carrier 59SC5A is a Limit Circuit Fault. A limit switch or flame rollout switch has opened during furnace operation. The control board responds by running the blower for 4 minutes or until the switch closes again, whichever is longer — this cools the heat exchanger back to a safe temperature.

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

The overwhelming majority of Code 33 events are caused by restricted airflow. A dirty or clogged air filter is the most common cause by far. Blocked or closed supply registers and return vents are the next most likely culprit. Less common causes include a loose blower wheel, a failing blower motor or capacitor, or vent sizing issues. If the flame rollout switch is what triggered this code, that is a more serious condition — the rollout switch requires manual reset (press the button on the switch) and may indicate a blocked flue or cracked heat exchanger.

If 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 you cannot see light through it easily, the filter is too dirty and needs to 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. Clear the area around the furnace Make sure nothing is stored against or on top of the furnace. The unit needs adequate clearance for proper air circulation.
5. Restore power and monitor Turn the gas supply back on, then turn on the breaker. Set the thermostat to call for heat. Monitor the furnace for 10-15 minutes. It should complete a full heating cycle without the limit switch tripping 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 (identified by a manual reset button) — may indicate a cracked heat exchanger or blocked flue
• The blower motor sounds weak, makes grinding noises, or does not reach full speed
• The furnace short-cycles repeatedly (runs a few minutes then shuts off)
• You detect a burning smell while the furnace operates