Quick answer: Carrier code 33 (three long flashes, three short) is a limit circuit fault. One of the furnace’s overheat safety switches opened: the main high limit, the flame rollout switch or the draft safeguard switch. The most common cause is restricted airflow from a dirty filter, but a weak blower, a dirty blower wheel or a venting problem can also cause it. If it repeats, the board will move to code 13, a limit lockout.
Safety first: if you smell gas, do not reset or relight anything. Leave the home and call your gas utility (Nicor Gas 1-888-642-6748, or North Shore Gas 1-866-556-6005) or 911 from outside.
What Trips the Limit Circuit
- A clogged air filter, the cause in a large share of the code 33 calls we run.
- Closed or blocked supply registers and returns blocked by furniture.
- A dirty blower wheel or a weak blower motor that cannot move enough air.
- Undersized ductwork on a furnace that was upsized at replacement.
- A tripped flame rollout switch, which can mean blocked venting or a cracked heat exchanger.
- A failed limit switch that opens too early.
Safe Checks You Can Do
- Replace the filter.
- Open all supply vents and make sure returns are clear.
- Reset power once. Let the furnace run and watch whether the code returns.
If a rollout switch has a red reset button on top that has popped, do not keep pressing it. A rollout trip means flame came out of the burner area, and the cause needs to be found first.
What a Technician Checks
We measure the temperature rise across the furnace and the static pressure in the ducts, which tells us whether airflow is the problem. We inspect the blower wheel and motor, test the limit and rollout switches, and inspect the heat exchanger and venting when a rollout switch is involved.
What Code 33 Repairs Cost
| Repair | Flat-rate price |
|---|---|
| High limit switch replacement | $340 |
| Flame rollout switch replacement | $325 |
| Blower assembly removal and deep cleaning | $390 |
| Blower wheel replacement | $865 |
| Blower motor replacement | $830 to $2,220 |
Prices are flat-rate with parts and labor, as of November 2026. The $75 diagnostic is waived when you approve the repair, and Comfort Club members save 10 to 15% on repairs. Exact pricing depends on your furnace model and whether the part is in stock on our trucks.
Seeing a Carrier code in the Northwest Suburbs?
Pure Comfort Heating and Cooling services every major furnace brand from our shop in Streamwood, IL. Diagnostics are $75, or $99 on evenings, weekends and holidays, and the fee is waived when you approve the repair. You approve a flat-rate price before any work starts, and phones are answered 24/7.
Other Error Code Guides
- Furnace blinking light codes by brand
- Lennox furnace error codes
- Carrier, Bryant and Payne furnace codes
- Goodman, Amana and Daikin flash codes
- Trane and American Standard blink codes
- Furnace pressure switch errors
Frequently Asked Questions
Can a dirty filter cause Carrier code 33?
Yes. A clogged filter is the most common cause. It restricts airflow, the heat exchanger overheats and the high limit switch opens.
What is the difference between code 33 and code 13?
Code 33 is a limit circuit fault on one cycle. Code 13 is a lockout after the limit opened repeatedly or stayed open too long. Code 13 usually clears after about three hours.
Is code 33 dangerous?
The switch shutting the furnace down is the safety working. A repeated rollout trip can point to blocked venting or a cracked heat exchanger and should be inspected before continued use.
We diagnose and repair furnaces from our shop in Streamwood across Streamwood, Bartlett, Hoffman Estates, Schaumburg, Elgin, Hanover Park, Roselle, Arlington Heights and the rest of the Northwest and West Suburbs. See our furnace repair page for flat-rate prices.