Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Lennox Rooftop Unit Error Codes: Complete Technician Guide
Lennox commercial RTUs — including the LGH, LCH, LCA, and Strategos series — communicate faults via LED flash codes on the IFC board and, on iComfort-equipped units, alphanumeric codes on the communicating thermostat. This guide covers all common Lennox RTU fault codes.
How to Read Lennox RTU Flash Codes
The STATUS LED on the furnace control flashes a two-digit code: the first group of flashes is the tens digit, the second group is the units digit. A 3-3 pattern means code 33.
Lennox RTU Flash Code Table
| Flash Code | Fault | Common Cause |
|---|---|---|
| 1-1 | System lockout | Manual reset required |
| 1-2 | Blower fault | Blower motor or wiring issue |
| 1-3 | Pressure switch stuck open | Inducer issue, blocked flue |
| 2-1 | Gas heating lockout | 3 failed ignition attempts |
| 2-2 | Limit switch fault | Overtemperature — check airflow |
| 2-3 | Flame sense fault | Dirty sensor, bad ground |
| 2-4 | Ignition failure | No gas, bad igniter |
| 3-2 | Low-pressure switch open | Low refrigerant, dirty filter |
| 3-3 | High-pressure switch open | Dirty condenser, overcharge |
| 4-1 | Rollout switch open | Heat exchanger issue |
| 4-2 | Inducer motor fault | Failed motor or capacitor |
| 5-1 | Control board fault | Replace IFC board |
| 5-2 | EEPROM fault | Replace control board |
Lennox iComfort Alphanumeric Codes (LCA/LGH with iComfort)
| Code | Description | Action |
|---|---|---|
| 103 | Pressure switch stuck open | Check inducer, flue, pressure switch |
| 111 | Low-pressure lockout | Check refrigerant charge |
| 114 | High-pressure lockout | Check condenser and charge |
| 125 | Defrost fault | Check defrost sensor and board |
| 204 | Blower fault | Check motor and capacitor |
| 223 | Flame sense fault | Clean/replace flame sensor |
| 225 | Ignition failure | Gas, igniter, sensor |
| 231 | Limit switch open | Airflow restriction |
| 327 | Communication fault | Check wiring between boards |
| 332 | Outdoor sensor fault | Replace sensor |
| 411 | Gas valve fault | Check valve and wiring |
| 412 | Rollout switch open | Heat exchanger check required |
| 432 | Inducer fault | Check inducer motor |
| 434 | Pressure switch fault | Check hoses and switch |
Most Common Lennox RTU Faults
2-2 — Limit Switch Fault
Restricted airflow is the cause in most cases:
- Replace dirty filter
- Check all supply and return grilles are open
- Inspect blower wheel for debris
- Verify limit switch is not defective (check continuity)
3-3 — High Pressure
Wash condenser coil. Confirm condenser fan motors are operating. Check refrigerant subcooling: target 10–15°F for R-410A units.
2-4 — Ignition Failure
- Check gas supply pressure at the manifold
- Inspect hot surface igniter (replace if cracked or over 5 years old)
- Clean flame sensor with fine steel wool
3-2 — Low-Pressure Lockout
Check refrigerant charge with gauges. Inspect evaporator coil for ice formation. Check TXV operation.
Lennox RTU Parts Reference
| Part | Notes |
|---|---|
| Hot surface igniter | 18K-ohm type — model-specific |
| Flame sensor | Check µA output — must exceed 1.5 µA |
| Run capacitor | Test with capacitor meter before condemning motor |
| Pressure switch | High or low side — match OEM pressure setting |
| IFC control board | Cross-reference by model and serial |
| Condenser fan motor | Check RPM and HP spec |
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Pro tip: Lennox RTU iComfort-equipped units log fault history accessible via the iComfort thermostat menu under Settings > Advanced > Diagnostics. This gives a timestamp of each fault.