Carrier furnace Code 33 — Limit or flame rollout switch open

What this error means

A safety circuit opened: either the high-temperature limit (the furnace overheated — almost always an airflow problem like a clogged filter, closed registers, or a dirty coil) or the flame rollout switch (flames escaped the burner box — a genuine combustion safety event). The distinction matters: limit trips are usually a filter change; a tripped rollout switch has a manual reset button and warrants professional inspection before you reset anything.

Symptoms

• Furnace starts then shuts down mid-cycle
• Code 33 on the status LED
• Common right after switching to a high-MERV (restrictive) filter

Try this first (safe DIY steps)

1. Replace the filter with the correct (not overly restrictive) type and make sure returns/registers are open
2. Power off 30 seconds and restore
3. If the code returns immediately and you find a small button on a switch near the burner box (rollout switch), do NOT just keep resetting it — that's a combustion safety device

Safety: power off before any physical intervention. Never bypass covers or interlocks — fuser areas run hot enough to burn.

When to call a technician

A repeating 33 after a fresh filter = blower or coil inspection ($150–$400 typical if a limit switch needs replacement). Any tripped rollout switch = professional combustion inspection, full stop.