Carrier furnaces report faults with an LED on the control board, usually visible through a small window in the blower door. Most modern Carrier and Bryant boards flash a two-digit code. Here is how to read it, what the common codes mean, and which ones are safe to check yourself.
How to Read a Carrier Furnace Error Code
Modern Carrier and Bryant control boards flash two-digit codes. Short flashes give the first digit, then long flashes give the second digit. For example, 1 short flash followed by 4 long flashes is code 14. A steady LED means normal operation, and rapid flashing means the line-voltage polarity is reversed. Codes vary by model and board, so the code chart on the inside of the furnace door is the authoritative reference. On communicating Infinity systems, the thermostat can also show a fault description.
Carrier Furnace Flash Code Reference
| Code | Fault Description | Common Cause | Action |
|---|---|---|---|
| Steady On | Normal operation | No active fault | No action needed |
| Rapid Flash | Reversed line-voltage polarity | Wiring fault at the furnace power supply | Call a technician or electrician |
| 13 | Limit circuit lockout | Repeated limit trips, often from restricted airflow | Replace the filter and check vents; call if it returns |
| 14 | Ignition lockout | Failed ignition attempts: gas supply, igniter, or flame sensing problem | Confirm the gas valve is open and power-cycle once; call if it returns |
| 31 | Pressure switch did not close | Inducer motor problem, blocked vent or intake, or pressure hose issue | Check outdoor vent pipes for snow or ice; otherwise call |
| 33 | Limit or rollout switch open | Overheating from restricted airflow, or flame rollout | Replace the filter; call if it repeats or a rollout switch has tripped |
| 34 | Ignition proving (flame sense) failure | Dirty or failing flame sensor, gas pressure, or burner problem | Call a technician |
| 41 | Blower motor fault | Failed blower motor or motor control | Call a technician |
| 42 | Inducer motor fault | Failed inducer motor or wiring | Call a technician |
| 45 | Control board fault | Failed furnace control board | Call a technician |
Code 33: The Most Common Carrier Fault in Ontario Winters
Code 33 means a limit or rollout switch is open, and it is the most common winter service call we receive for Carrier furnaces. In most cases the high-limit switch has tripped because a dirty air filter restricts airflow and the heat exchanger overheats. Replace the filter first. If code 33 comes back, the blower may be failing or a return air duct may be blocked, and a technician should also rule out a tripped rollout switch.
Carrier Model-Specific Notes
- Carrier Comfort and Performance series: Use two-digit LED flash codes like those above. Check the chart on the blower door for your model
- Carrier Infinity (such as 59MN7): Communicating system. The thermostat can show fault descriptions, and control or communication faults need specialized diagnostic tools
- Carrier Greenspeed systems: Variable-capacity equipment where fault diagnosis needs a Carrier-trained technician
When You Can Fix It Yourself vs. When to Call
Safe DIY: replace the air filter (code 13 or 33), clear snow or ice from the outdoor vent pipes (code 31), confirm the gas valve is open (code 14), and power-cycle the furnace once. Flame sensor cleaning, igniter, inducer, blower, gas valve and control board work, and anything involving the heat exchanger or a rollout switch, requires a TSSA-licensed technician with proper diagnostic equipment.
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Book Same-Day Carrier Furnace Repair?Frequently Asked Questions
My Carrier furnace is flashing code 33: what does that mean?
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