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.

What does Lennox error code E201 mean on an SL280 or SLP99 furnace?

E201 indicates the integrated furnace control (IFC) has lost communication with the variable-speed indoor blower motor. The communicating ECM blower exchanges digital signals with the IFC; when that link drops, the board logs E201 and disables heating to prevent unventilated heat buildup. Cycle furnace power for 60 seconds to clear transient dropouts; if E201 returns, inspect the multi-pin communication harness between the IFC and the blower-motor module for loose pins, damaged insulation, or moisture, and check the IFC's 3-amp or 5-amp fuse. Persistent faults usually mean an ECM motor module failure that requires replacement and capacity-code reprogramming.

What does E290 mean on a Lennox furnace and how do I fix it?

E290 is an ignitor circuit fault. The IFC monitors the hot-surface ignitor before and during warm-up; an open or out-of-range resistance reads E290 and forces a soft lockout for about an hour. Power down the furnace, disconnect the ignitor harness, and measure resistance across the ignitor terminals — 39 to 70 ohms is acceptable, OL confirms a failed silicon-nitride element. Inspect for visible cracks or carbon tracking, check harness continuity from the IFC ignitor output to the ignitor connector, and verify 120 V AC is present at the ignitor terminals during the warm-up period. If voltage is missing, the IFC's ignitor triggering circuit is faulty; if voltage is present and resistance is in spec but E290 returns, a board-level sensing circuit failure is the most likely cause.

What is Lennox error code E270 and why is the furnace in Watchguard mode?

E270 is a soft lockout triggered after the furnace has exceeded the maximum number of ignition retry attempts without establishing a flame. The IFC enters Watchguard mode for approximately one hour before allowing another ignition attempt. Common root causes: a closed manual gas-shutoff valve or empty propane tank, a failed or cracked hot-surface ignitor, a mispositioned ignitor not aligned with the gas stream, a dirty flame sensor that does not confirm ignition quickly enough, or low gas-manifold pressure preventing flame establishment. Verify gas supply, wait out the Watchguard period or power-cycle the furnace for 30 seconds, then clean the flame sensor with fine steel wool and confirm ignitor resistance is 39 to 70 ohms before the next ignition attempt.

How do I troubleshoot a Lennox E274 limit switch lockout?

E274 is a soft lockout on Lennox communicating furnaces (SL280, SLP99, EL296, ML296). It fires when the primary limit switch opens for more than three minutes across multiple ignition attempts — the furnace is overheating. The single most common cause is a clogged air filter starving the heat exchanger of return airflow; replace the filter first. Then walk the home and confirm every supply and return register is fully open, check zone-damper actuators on iHarmony or Smart Zoning systems for stuck-closed positions, measure gas-manifold pressure against the nameplate (overfiring trips the limit independently of airflow), and clear any blockage in the condensate drain or cold-end heater box on 90%+ models. If the limit continues to trip after these checks, a cracked heat exchanger is possible and requires combustion analysis by a licensed technician — carbon-monoxide hazard.

What does E225 or E227 mean on a Lennox two-stage furnace?

E225 means the high-pressure switch failed to close when the inducer ramped to high-fire draft (preventing second-stage operation), while E227 means the low-pressure switch closed at the start of an ignition trial but then dropped open mid-cycle (a draft interruption). Both point to combustion-air inducer or vent-system problems. Inspect intake and exhaust PVC for blockages, ice, or collapsed sections; clear the condensate trap and the cold-end heater box on condensing models; check the pressure-switch hose for kinks, trapped condensate, or collapse under operating pressure; and measure inducer draft with a manometer at the housing tap, comparing against the switch closing setpoints listed in the SL280DFNV / SLP99UHV IOM. A draft that starts adequate and falls during the heat cycle indicates a thermal restriction or a failing inducer motor capacitor that only manifests under load.

Quick Answer

Lennox communicating furnaces and air handlers display three-digit E-codes (E200 series for ignition and pressure-switch faults, E270/E290 for ignitor and flame issues, E274/E292 for blower and limit faults, E370/E402 for accessory interlocks) on the iComfort S30 / E30 / M30 thermostat. The same code book is shared across Aire-Flo, Concord, and Ducane (Lennox-owned sister brands).

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By Octavian Ghindea

Co-founder & CTO · Reviewed May 2026

What error codes does Lennox display?

E201

Indoor blower communication failure — check IFC-to-ECM-motor harness, fuse, and motor module

E270

Soft lockout after maximum ignition retries — verify gas supply, ignitor, and flame sensor

E274

Primary limit switch open soft lockout — replace air filter and confirm register / damper airflow

E290

Ignitor circuit fault — measure ignitor resistance (39–70 ohms) and IFC output voltage

E292

Indoor blower motor unable to start — check for seized wheel, obstruction, or failed ECM module

E200

Hard lockout from flame rollout — inspect heat exchanger, vent, and inducer before reset

E225

High-pressure switch failed to close — inspect vent restriction and inducer draft

E227

Low-pressure switch open during ignition / run — check vent termination and condensate trap

E250

Primary limit circuit open during ignition — verify temperature rise and gas-manifold pressure

E370

Condensate-pump interlock open for over 2 minutes — check pump float switch and drain line

Common questions about Lennox equipment

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