Carrier 33 Error Code — What It Means
Carrier furnace fault code 33 means the limit device is open — the high-limit or rollout switch has tripped and cut off gas to prevent overheating. The control board flashes 3 long, 3 short. This is a safety lockout: the furnace detected that heat exchanger or plenum temperatures exceeded safe limits, or combustion gases rolled out of the heat exchanger. The root cause is almost always an airflow or combustion problem, not a failed limit switch itself.
Common Causes
- Restricted airflow — Clogged air filter, blocked return, or collapsed duct forces the heat exchanger to overheat. Most common cause on residential units.
- Blower motor failure or weak capacitor — If the blower doesn’t spin at full speed, heat builds up faster than it can be moved. Check amp draw against nameplate.
- Dirty heat exchanger — Carbon buildup or scale reduces heat transfer and drives surface temps up until the limit trips.
- Rollout switch tripped — Cracked heat exchanger or blocked flue lets combustion gases escape the firebox, tripping the rollout switch (separate from the high-limit).
Step-by-Step Fix
- Check and replace the air filter — Pull the filter and inspect it. If it’s grey or clogged, swap it. A $5 filter causes 40% of limit lockouts. Restart after replacement.
- Verify blower operation — With the furnace running, confirm the blower starts within 30–60 seconds of ignition. If it doesn’t spin or spins slowly, test the run capacitor with a capacitor meter. Replace if it reads more than 10% below rated capacitance.
- Inspect supply and return vents — Walk the house and open all closed registers. Check that furniture or boxes aren’t blocking return air grilles. Blocked returns are a fast path to limit trips.
- Locate and reset the rollout switch — The rollout switch sits near the burners. It has a red reset button. Press it firmly. If it trips again within one cycle, stop — you likely have a cracked heat exchanger and need a tech.
- Reset the system — Cut power at the furnace switch for 30 seconds, then restore. If code 33 returns within the first heating cycle after fixing airflow, escalate to a heat exchanger inspection.
Parts Often Needed
| Part | Notes |
|---|---|
| High-limit switch | Trips on temp; test with multimeter for continuity at room temp — should be closed |
| Rollout limit switch | Manual reset; if it trips repeatedly, the heat exchanger is the problem |
| Blower run capacitor | Most common blower fix; costs $10–20 and takes 15 minutes |
| Air filter (MERV 8–11) | Replace every 90 days minimum |
Technician Tips
When diagnosing Carrier error codes, start with the LED blink count on the furnace control board. The green LED tells a different story than the amber one. A single blink usually means power only — no fault. Two to four blinks point to ignition or pressure switch issues. Five or more blinks often indicate limit circuit or sensor failures. Always clean the flame sensor with emery cloth before replacing any parts — it fixes roughly 40% of Carrier ignition lockout calls.
Common replacement parts for Carrier systems:
- Carrier flame sensor (part #HH18ZA641)
- Limit switch kit (part #HH18HA600)
- Pressure switch (part #HH18HA719)
Need Replacement Parts?
Finding the right replacement parts for your equipment is critical to getting it running again. Check current prices on Amazon — prices and availability change frequently.
When to Call a Pro
If the rollout switch trips more than once after resetting, or if you find cracks or rust holes in the heat exchanger during inspection, stop using the furnace and call a tech. A cracked heat exchanger is a carbon monoxide hazard.