HVAC Code Lookup

Carrier Furnace Code 33

Soft lockoutShown as: 3 short flashes, 3 long flashes

Limit circuit fault. The high-limit switch, draft safeguard, flame rollout, or blocked-vent shutoff switch is open - the furnace is overheating or venting badly. The blower runs for 4 minutes or until the switch remakes. If the switch stays open more than 3 minutes the code changes to lockout 13.

Manual wording: “33 LIMIT CIRCUIT FAULT - Indicates the limit, or flame rollout is open. Blower will run for 4 minutes or until open switch remakes whichever is longer.”

Documented for: 58SC0A, 58SC0B, 58MCB, 58MVP, 58UVB

Likely causes, most common first

  1. Dirty filter or restricted duct system
  2. Loose blower wheel
  3. Defective switch or connections
  4. Defective blower motor or capacitor
  5. Inadequate combustion air supply (flame rollout switch open)
  6. Restricted vent or improper vent sizing
  7. Excessive wind
  8. Improper gas input adjustment

What you can safely check yourself

  • Check and replace the air filter - a clogged filter is the most common cause
  • Make sure supply registers and return air grilles are open and unblocked
  • Confirm the furnace door is fully seated
  • Check outdoor vent terminations for blockage

When to call a technician

If the code returns after a filter change, the limit or rollout switch is tripping for a reason a technician must diagnose. The flame rollout switch requires manual reset and must never be reset without finding out why it tripped.

A Carrier furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.

Sources

Other Carrier codes