Lennox AC Error Codes: Diagnosis Guide
When your Lennox air conditioner stops cooling or shuts itself off unexpectedly, the control board is logging a fault code that tells a trained technician — and an informed homeowner — exactly where the problem lies. Lennox AC systems communicate faults through LED codes on the outdoor unit control board or through alphanumeric codes on an iComfort or communicating thermostat. This guide covers the most common Lennox air conditioner error codes and what you can do about them.
Where to Find Lennox AC Error Codes
On standard Lennox split-system air conditioners, the control board is located inside the outdoor condenser unit. A blinking LED indicates a fault code. On iComfort-enabled systems (XC20, XC21, XC25), fault codes appear on the iComfort S30 or E30 thermostat screen and are logged with timestamps. You can access fault history on the thermostat under Settings > Diagnostics.
Common Lennox AC Error Codes
| Code / Pattern | Meaning | What to Do |
|---|---|---|
| E1 | High-pressure lockout — refrigerant circuit fault | Check outdoor unit for blocked airflow; call tech |
| E2 | Low-pressure lockout — possible refrigerant leak or low charge | Technician required — refrigerant handling |
| E3 | High discharge temperature | Check condenser coil for debris; call if clean |
| E4 | Communication fault — thermostat/air handler disconnect | Check thermostat wiring; call if wiring appears intact |
| E6 | Outdoor coil sensor fault | Technician required — sensor replacement |
| E8 | Indoor coil (evaporator) sensor fault | Technician required |
| E9 | Compressor overload — high current draw | Technician required — compressor or capacitor issue |
| 2 flashes | System lockout — repeated high-pressure trips | Check outdoor unit; call for service |
| 3 flashes | Low-pressure fault | Technician required — check refrigerant charge |
| Fault 25 | Inverter fault (variable-speed models) | Technician required — inverter board |
What Homeowners Can Check
Outdoor unit airflow: Clear grass clippings, leaves, and debris from the sides and top of the outdoor condenser. The condenser coil fins should be visible and relatively clean. A blocked condenser causes high-pressure faults (E1, E3) by trapping heat that should be expelled outdoors.
Air filter: A severely clogged indoor air filter reduces airflow across the evaporator coil, causing it to freeze and triggering low-pressure codes. Replace the filter and let the ice melt (run the fan only for 2-3 hours) before restarting cooling.
Circuit breaker: Air conditioners draw high current at startup. If the outdoor unit is not running at all, check for a tripped breaker — both the main panel breaker and the disconnect box near the outdoor unit.
Thermostat settings: Confirm the thermostat is in COOL mode, the set temperature is below the current room temperature, and the system mode is not set to OFF or FAN ONLY.
Disconnect box: Some technicians inadvertently leave the outdoor unit disconnect in the OFF position after a service call. Check that the pull-out fuse block in the disconnect box mounted near the condenser is fully seated.
When to Call a Professional
Low-pressure faults (E2, 3 flashes) almost always indicate a refrigerant leak or undercharged system. Refrigerant is a regulated substance that requires an EPA Section 608 certification to handle — it cannot be topped off by a homeowner. Compressor faults, inverter faults, and repeated high-pressure trips all require professional diagnosis with manifold gauges and specialized equipment.
If your Lennox AC is tripping on the same fault repeatedly, avoid the temptation to keep resetting it. Running an air conditioner in fault is one of the fastest ways to damage an expensive compressor.
Burkhardt Heating, Cooling, Plumbing & Electric provides same-day AC repair service throughout Southeastern Wisconsin, with NATE-certified technicians and 24/7 emergency availability.
Lennox AC Repair in Southeastern Wisconsin
A fault code is your AC system asking for help before the situation gets worse. Responding quickly — especially early in the cooling season — keeps small repairs from becoming compressor replacements.
If you see any of these codes, call Burkhardt at 414-206-3049. Our NATE-certified technicians can diagnose and repair the issue — usually in a single visit.
Ready to schedule service? Call Burkhardt Heating, Cooling, Plumbing & Electric at 414-206-3049 or contact us online. Available 24/7 for emergencies.
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