Carrier 58MVP Error Code Code 33: Limit or Flame Roll-Out Switch Is Open

What Does Code Code 33 Mean?

Code 33 indicates that one of the furnace's high-temperature safety switches has opened. This could be the main limit switch, the flame rollout switch, or the auxiliary limit switch. These switches cut off gas flow to the burners when they detect temperatures exceeding safe levels, preventing heat exchanger damage and potential fire hazards.

On the 58MVP, code 33 can also indicate that the unit has shifted into high-heat-only mode after 2 successive low-heat limit trips. This is a protective behavior — the control decides that low-heat operation is causing overheating (often due to insufficient airflow at the lower blower speed) and forces the furnace to run at high heat only, where the blower runs faster and moves more air.

The most common cause is a dirty air filter restricting airflow, followed by blocked registers or ductwork restrictions. If the flame rollout switch has tripped (indicated by a popped-out red reset button on the switch), this points to a more serious combustion issue — flames are rolling out of the combustion chamber rather than being drawn through the heat exchanger properly. Do not reset the rollout switch without professional inspection.

Common Causes

How to Fix It: Check Air Filter and Registers

What You'll Need

• Replacement air filter (if needed)
• Flashlight

Steps

1. Turn off power at the breaker or power switch AND shut off the gas supply Locate the furnace circuit breaker and turn it OFF. Find the gas shutoff valve on the supply line to the furnace and turn it to the closed position (perpendicular to the pipe). If you smell gas, leave immediately and call your gas company.
2. Check and replace the air filter Locate the air filter (typically at the bottom of the furnace or in the return air duct). Pull it out and inspect. If it is visibly dirty, clogged, or you cannot see light through it, replace it with a new filter of the same size. This is the most common fix for code 33.
3. Open all supply and return registers Walk through every room in the house and make sure all supply registers and return air grilles are fully open and not blocked by furniture, rugs, or curtains. Restricted airflow at the register level has the same overheating effect as a dirty filter.
4. Restore power and gas, then test Turn the gas supply valve back to the open position (parallel to the pipe). Turn the breaker back ON. Set your thermostat to call for heat and observe the furnace through a full heating cycle.

When to Call a Professional

Contact a licensed HVAC technician if:

• Code 33 returns after replacing the filter and opening all registers
• The flame rollout switch has tripped (red button popped out) — do not reset it without professional inspection
• The furnace is stuck in high-heat-only mode and won't return to normal two-stage operation after fixing airflow
• You notice a burning or overheating smell from the furnace
• The limit switch or auxiliary limit switch appears damaged or does not reset
• Code 33 keeps escalating to code 13 (limit switch lockout)