Carrier furnace Status code 33: Limit circuit fault — a limit or flame rollout switch is open

Code 33 means a limit or flame rollout switch is open. The blower runs for four minutes or until the switch remakes, and if it stays open beyond three minutes the code escalates to lockout 13.

Applies to: Carrier 58MCB four-way multipoise condensing gas furnaces and related models using the same integrated furnace control. Bryant and Payne furnaces of the same generation use the same control and code set.

What it means

This code covers two switches with very different implications. A high limit opening means the furnace is overheating, which is usually an airflow problem and usually mundane. A flame rollout switch opening means flame escaped the burner compartment, which is serious. Carrier's documentation groups them because they share a circuit, and it notes the critical operational detail: the flame rollout switch requires manual reset. So if the furnace recovers on its own, a limit was involved; if it stays down until someone presses a button, rollout was. That distinction is worth establishing before deciding how urgently to act.

Most common causes

• A dirty filter or restricted duct system, which Carrier lists explicitly.
• A restricted vent.
• Improper vent sizing.
• Excessive wind.
• A loose blower wheel.
• A defective blower motor or capacitor.
• A defective switch or its connections.
• Inadequate combustion air supply, which Carrier specifically associates with the flame rollout switch opening.

How to diagnose it

1. Check the air filter first. Carrier lists it among the causes and it is the most common.
2. Establish whether a rollout switch is involved. If the furnace will not restart until a button is pressed, it is rollout and the situation is more serious.
3. Note whether the blower runs for four minutes after the fault, which matches Carrier's documented behaviour.
4. Walk the house and check for closed or blocked registers and a clear return grille.
5. Note whether the code escalates to 13, which means the circuit stayed open beyond three minutes.

How to fix it

1. Replace the air filter and open closed registers. This resolves a large share of limit circuit faults.
2. If a flame rollout switch has tripped, stop and call a technician. Do not press the reset button — the switch tripped because flame went somewhere it should not, and resetting without finding out why puts the furnace straight back into that condition.
3. Blower motor faults, a loose blower wheel, duct problems, and vent restrictions are technician work.
4. Have working carbon monoxide alarms on every level of the home and outside each separate sleeping area, which is what NFPA 720 and US Consumer Product Safety Commission guidance call for. That includes the level the furnace is on — basements, attics and utility spaces are where a combustion fault starts, and an alarm only on bedroom floors can miss it for a long time. This fault is one of the situations those alarms exist for.

Notes

The manual reset on the flame rollout switch is the clearest signal in this code. Every other safety in the circuit resets itself; that one requires a person, precisely so the condition gets investigated.

Related

• Status code 13 — Limit circuit lockout
• Status code 31 — Pressure switch did not close

All codes for this manufacturer

Carrier, Bryant and Payne furnace status codes — every code, with what differs between models.

Sources

• Carrier 58MCB Service Manual, 327884-101 Rev. B — status codes