Carrier furnaces use LED flash codes to communicate faults on the control board. The number of flashes tells you the fault category, and Carrier's control boards show both a flash code and often a two-digit fault code on newer Infinity series models. Here is a complete reference for every Carrier furnace fault code used on 58MCA, 58CVA, 59MN7, and Infinity 98 models.
How to Read a Carrier Furnace Error Code
On standard Carrier Performance and Comfort series furnaces: the LED flashes a short pattern, pauses, then repeats. Count the flashes before the pause. On Carrier Infinity series with a communicating control, the furnace control board shows a two-digit code (like 24 or 31) and the Infinity Touch thermostat displays a full fault description. Most fault history can be retrieved by pressing the diagnostic button on the control board.
Carrier Furnace Flash Code Reference
| Flash Code | Fault Description | Common Cause | Action |
|---|---|---|---|
| 1 Flash | Rollout switch open | Flame escaping combustion chamber - cracked heat exchanger | SHUT OFF furnace - call technician immediately |
| 2 Flashes | Pressure switch fault | Blocked flue vent, failed inducer motor, cracked pressure hose | Check vent for blockage; call if problem persists |
| 3 Flashes | High-limit switch open | Restricted airflow - dirty filter, blocked vents, slow blower | Replace filter; check all vents are open |
| 4 Flashes | Open limit device - thermal protection | Overheating from restricted airflow | Check filter and blower; call if recurring |
| 5 Flashes | Flame not sensed | No gas, dirty flame sensor, or failed igniter | Check gas supply; clean flame sensor |
| 6 Flashes | Inducer pressure fault | Inducer motor issue or pressure switch stuck open | Inspect inducer; call for diagnosis |
| 7 Flashes | Gas valve open fault | Gas valve stuck open - safety issue | Call technician immediately |
| 8 Flashes | Low flame sense | Dirty flame sensor or weak gas pressure | Clean flame sensor; check gas pressure |
| Continuous Flash | Normal operation | System running - no fault active | No action needed |
Carrier Infinity Series Fault Codes (Two-Digit)
Carrier Infinity series furnaces (59MN7, 58CVA) with communicating controls show two-digit fault codes on the control board and full descriptions on the Infinity Touch thermostat. The most common codes are: 13 (limit switch fault), 24 (secondary voltage fuse open), 31 (pressure switch stuck open), 33 (limit switch fault - airflow), 34 (ignition failure), and 41 (inducer motor fault).
Code 33: The Most Common Carrier Fault in Ontario Winters
Code 33 (or a 3-flash on standard models) is the high-limit switch fault - and it is the most common winter service call we receive for Carrier furnaces. In most cases it is caused by a dirty air filter that has not been replaced in 3 months or more. The heat exchanger overheats, the safety limit trips, and the furnace shuts down. Replace the filter first. If it trips again, the blower may be failing or a return air duct may be partially collapsed.
Carrier Model-Specific Notes
- Carrier Comfort 80 (58MCA): Uses LED flash codes. Most common failures are igniter (5-flash) and high-limit (3-flash)
- Carrier Performance 96 (59TP6, 59SP5): Uses LED flash codes. Inducer and pressure switch faults (2-flash, 6-flash) are more common on two-stage models
- Carrier Infinity 98 (59MN7): Communicating system with digital fault codes. Control board and communicating thermostat faults require specialized diagnostic tools
- Carrier Greenspeed (59MN7, 25VNA): Variable-speed compressor and modulating gas valve - fault diagnosis requires Carrier Service Technician Software (STS)
When You Can Fix It Yourself vs. When to Call
Safe DIY: Replace the air filter (3-flash limit fault), clean the flame sensor with fine steel wool (5 or 8-flash), and confirm the gas valve is open (5-flash). Everything involving the heat exchanger (1-flash rollout), inducer motor, gas valve, or communicating control boards requires a TSSA-licensed technician with proper diagnostic equipment.
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