What does Lennox error code E4 mean?

E4 on Lennox systems indicates a high-pressure lockout. The refrigerant circuit high-pressure switch has tripped. Check condenser coil cleanliness, condenser fan operation, refrigerant overcharge, and ambient temperature. The system will auto-reset after the pressure drops, but repeated trips indicate a persistent issue requiring investigation.

How do I access diagnostic codes on a Lennox iComfort thermostat?

On the Lennox iComfort S30 or E30 thermostat, navigate to Settings > Advanced Settings > Dealer Controls > Diagnostics. Active faults are displayed with descriptions. You can also run a system test from this menu to verify all components are operational. Ensure the thermostat firmware is up to date before diagnosing software-related issues.

What causes a Lennox furnace to show a code 33 fault?

Code 33 on Lennox furnaces indicates a limit device fault — the high limit switch or rollout switch has opened. This is often caused by restricted airflow (dirty filter), blocked flue or combustion air intake, oversized furnace, or a failed limit switch. Always check the air filter first — a clogged filter is the most common cause.

How do I check refrigerant charge on a Lennox XC21 system?

The Lennox XC21 uses R410A and requires charging by weighing refrigerant, not by superheat/subcooling alone for the initial charge. Use the charging chart on the inside of the access panel as a reference. Target 10-12°F subcooling at the condenser outlet and 8-12°F superheat at the evaporator outlet under rated conditions.

What does Lennox diagnostic LED code 6 blinks mean?

Six blinks on the Lennox control board diagnostic LED indicates a rollout switch fault. The rollout switch has detected abnormal flame rollout, which is a safety hazard. Do not reset and restart the furnace without inspecting the heat exchanger for cracks, verifying the combustion air intake is clear, and checking the flue for blockages.

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Common Lennox Error Codes

High pressure lockout — check condenser coil and refrigerant charge

Limit device fault — check airflow and heat exchanger

6 blinks

Rollout switch fault — inspect heat exchanger immediately

Communication error between indoor and outdoor units

Outdoor coil temperature sensor fault

Frequently Asked Questions — Lennox

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