Carrier 58MVC Error Code 33: Limit Circuit Fault

What Does Code 33 Mean?

Code 33 is the active limit-circuit fault on the 58MVC, and it is the fault that precedes lockout code 13. It appears when the high-limit or flame rollout switch is open, or when the furnace has switched itself to high-heat-only mode after two successive low- or medium-heat limit trips. When the fault is active, the variable-speed blower runs at medium-heat airflow for 4 minutes, or until the open switch remakes, whichever is longer.

The timing matters: if the limit stays open longer than 3 minutes, code 33 escalates to lockout code 13. If it reopens in under 3 minutes, code 33 keeps flashing until the blower shuts off. In other words, code 33 is what you catch the furnace doing while it is overheating; code 13 is what happens if it cannot cool down in time.

The most common cause is exactly what you would expect on a condensing furnace with a tightly matched blower and heat exchanger: a dirty filter or a restricted duct system starving the exchanger of airflow. Because this furnace modulates, two limit trips on low- or medium-heat push it into high-heat-only operation as a protective response, which is why the furnace may suddenly feel like it is blowing harder and hotter than usual.

Uncommon causes — a loose blower wheel, a stuck high-heat solenoid in the gas valve, a misaligned limit or limit shield, or a tripped flame rollout switch that needs manual reset — are technician tasks and are not homeowner-serviceable.

What You'll Notice

• The furnace runs, then the burners cut out while the blower keeps running to cool it
• The furnace suddenly seems to run only on its highest, hottest stage
• Short, repeated heating cycles that never reach the thermostat setpoint
• A visibly dirty filter or blocked return-air grille
• Warm air followed by a stretch of cool air from the registers

Common Causes

How This Is Diagnosed

The first and most productive check is airflow: inspect and replace the filter, then confirm supply and return registers are open and the return path is not blocked. Restricted airflow is the leading cause and the one a homeowner can fix.

If airflow is good and code 33 continues, a technician checks the rear air baffle, looks for a loose blower wheel, verifies the low- and medium-heat gas input, inspects the high-heat gas-valve solenoid, and confirms the limit and its shield are properly positioned. A tripped flame rollout switch requires a manual reset and a root-cause investigation, both of which are professional work.

How to Fix It: Clear the Airflow Restriction

What You'll Need

• Replacement furnace filter (correct size) 🛒 Find at FiltersFast · 🛒 Find at Amazon
• Flashlight

Steps

1. Turn off power at the furnace switch or breaker and shut off the gas supply Set the furnace disconnect to OFF (or trip its breaker) and close the manual gas shutoff valve before opening any panel. If you smell gas, leave the house immediately and call your gas company from outside.
2. Inspect and replace the filter Pull the filter and hold it to a light. If light does not pass through, it is choking airflow. Install a clean filter of the exact printed size with the airflow arrow pointing toward the furnace.
3. Open the return and supply path Make sure return-air grilles are not blocked by furniture or rugs and that supply registers throughout the house are open. This modulating furnace needs its full designed airflow to stay below the limit.
4. Check for obvious duct restrictions Look for closed dampers, a collapsed flex duct near the furnace, or a closed zone damper if your system is zoned. Reopen anything that is obviously closed off.
5. Restore power and observe staging Turn the gas back on, restore power, and cycle the furnace switch. Watch a full heating cycle to confirm the furnace modulates normally instead of jumping straight to high-heat.

When to Call a Professional

Contact a licensed HVAC technician if:

• Code 33 persists after a clean filter and confirmed open registers
• The furnace is stuck in high-heat-only mode after airflow is corrected
• You suspect a flame rollout switch tripped — it needs a manual reset and professional diagnosis
• The blower sounds like it is rubbing, wobbling, or vibrating

✓ Verified against manufacturer service manual — March 2026