Lennox SL280UHV Error Codes — Complete Fault Code Diagnostic Guide

Error Codes: Lennox SL280UHV

What it means: The Lennox SL280UHV is a high-efficiency two-stage variable-speed gas furnace that communicates diagnostic faults through a status LED on the control board (located behind the lower access panel). The LED blinks in a repeating pattern: count the flashes in the first group, then the second group — together they form the fault code. For example, 3 flashes, pause, 4 flashes = fault code 34. Single-digit codes blink once with a long pause between groups.

This unit uses the Lennox iComfort or conventional thermostat interface and is common in high-end residential and light-commercial installations from 2010–present.

Lennox SL280UHV Fault Code Reference

Code 11 — No Previous Code

Board has been powered with no faults. Normal status on a healthy system.

Code 12 — Blower On After Power Up

Blower ran at power-up to clear residual heat. Normal operation.

Code 13 — Limit Switch Fault (Soft Lockout)

The high-temperature limit switch tripped. Furnace locked out after three consecutive trips. Most common cause on SL280UHV units: dirty air filter, blocked return, or failed variable-speed blower capacitor.

Code 14 — Ignition Failure Lockout (Hard Lockout)

The board attempted ignition four times without establishing flame. Possible causes:

• Gas supply interrupted
• Hot surface igniter cracked or failed
• Flame sensor coated with oxidation
• Gas valve not opening

Reset: Cycle power off for 30 seconds.

Code 21 — Gas Valve Fault

Gas valve energized but no flame detected. Usually points to gas valve failure or, less often, an intermittent connection on the valve wiring harness.

Code 22 — Induced Draft Motor Fault

The inducer motor didn’t reach operating speed. Check capacitor first (cheap, fast fix). If motor hums without spinning, the capacitor is almost certainly failed.

Code 23 — Pressure Switch Did Not Open

The pressure switch didn’t open after the inducer shut down. Typically a stuck-closed pressure switch or condensate backup causing the switch to stay in the tripped position.

Code 24 — Secondary Voltage Fuse Blown

The 3A automotive-style fuse on the control board is blown. Almost always caused by a wiring short downstream — typically a shorted thermostat wire touching bare metal.

Code 25 — Control Board Fault

The board detected an internal fault. Power cycling often clears it; persistent code 25 = board replacement.

Code 31 — Pressure Switch Did Not Close

Inducer running but pressure switch won’t close. Classic causes: blocked condensate drain, cracked pressure switch hose, weak inducer motor, blocked flue or intake.

Code 32 — Pressure Switch Stuck Open After Ignition

Pressure switch opened during the heating cycle — usually a flue obstruction, condensate backup, or a pressure switch that’s drifted out of calibration.

Code 33 — Limit Switch Fault

The primary limit or auxiliary limit switch opened during the heating cycle. On the SL280UHV, the variable-speed blower should prevent this — a code 33 here usually means the blower’s ECM drive module is failing and not ramping up to full speed.

Code 34 — Ignition Proving Fault

Flame established but lost during the first heating cycle. Dirty flame sensor (clean with emery cloth), weak gas pressure, or drafting problem causing flame instability.

Code 41 — Blower Motor Fault

The ECM (electronically commutated motor) reported an error to the control board. Could be a motor winding failure, Hall sensor fault, or ECM drive module failure. This is one of the more expensive repairs on the SL280UHV — the ECM module alone runs $300–$600.

Code 42 — Inducer Motor Fault

Inducer motor current draw exceeded expected range. Could be a bad capacitor, failing bearings, or a partially blocked flue.

Code 43 — Low-Fire Pressure Switch Fault

SL280UHV-specific: the low-fire pressure switch failed to close during stage-1 operation. Check the low-fire pressure switch hose and switch.

Code 44 — High-Fire Pressure Switch Fault

High-fire pressure switch didn’t close when the board called for stage-2 operation. Same diagnostic path as code 43 but for the second pressure switch.

Code 45 — Control Board Memory Error

EEPROM checksum error on the control board. Power cycle first; persistent code = board replacement.

Common Causes of SL280UHV Faults

• Dirty air filter — Code 13, 33. SL280UHV has a high-MERV media filter that clogs faster than standard filters. Replace every 6–12 months.
• ECM blower issues — Code 41, 13. The variable-speed ECM motor is complex. A failing drive module ramps the motor down, reducing airflow and tripping limits.
• Condensate drain blockage — Code 23, 31, 32. This is a 90%+ AFUE condensing furnace that produces significant condensate. Drain line blockages are extremely common, especially in humid climates.
• Pressure switch hose cracks — Code 31, 32, 43, 44. The silicone hoses on SL280UHV pressure switches crack with age. A $2 hose causes a no-heat call.

Step-by-Step Fix {#step-by-step-fix}

1. Read the fault code. Remove the lower access panel. The LED is on the left side of the control board. Count flash groups. Write down the two-digit code.

2. Start with the filter. Pull the media filter (typically in the return air cabinet above the furnace or in the furnace itself). If it’s gray or blocked, replace it. A fresh filter resolves code 13 and 33 about 40% of the time.

3. Check the condensate drain. Locate the clear plastic drain line exiting the bottom of the furnace. Disconnect it at the outlet and blow through it — if you meet resistance, it’s clogged. Clear it with a wet/dry vacuum or compressed air. This resolves code 23, 31, and 32 in high-humidity installs.

4. Inspect the inducer and pressure switch hoses. With power off, trace all small-diameter rubber hoses from the inducer housing to the pressure switch(es). Look for cracks, kinks, or disconnected ends. Replace suspect hoses with matching-diameter tubing.

5. Clean the flame sensor. Remove the single screw holding the flame sensor rod in the burner assembly. Lightly polish the rod with #400 emery cloth until shiny. Reinstall. This clears code 34 in most cases.

6. Test the ECM blower. For code 41: with the furnace calling for heat, feel the airflow at a supply register during the blower on-delay. Airflow should ramp up smoothly over 30–60 seconds to full speed. Weak, erratic, or absent airflow with the motor energized suggests an ECM drive module failure.

7. Check the gas valve. For code 21: with the furnace in ignition attempt, listen for a soft click when the gas valve energizes. No click = bad valve solenoid. Click but no gas flow = stuck valve poppet.

8. Replace the control board fuse. For code 24: replace the 3A mini-blade automotive fuse on the board. If it blows again immediately, disconnect the thermostat wire at the furnace and re-energize — if fuse holds, the short is in the thermostat wiring.

Parts Often Needed {#parts-often-needed}

When to Call a Professional

Any fault involving the gas valve (code 21) or a hard lockout on ignition (code 14) that persists after cleaning the flame sensor should be diagnosed by a licensed HVAC tech. Gas valve replacement requires disconnecting the gas line and rechecking for leaks. The SL280UHV’s ECM blower system is also complex — drive module failures require specialized diagnostic tools to confirm before spending $400+ on parts.

Pro tip: The SL280UHV has a “last 5 fault codes” memory on the control board. Hold the diagnostic button (small pushbutton next to the LED) for 3 seconds — the board will blink out the last five fault codes in sequence. This is invaluable for intermittent faults that don’t repeat during a service call.

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