Lennox 103 Error Code — What It Means
Lennox error code 103 indicates a pressure switch fault — the iComfort or SureLight control board did not detect the pressure switch closing during the draft motor startup sequence. The furnace tries to prove inducer operation before allowing ignition, and when the pressure switch won’t confirm airflow, the ignition sequence is blocked. On Lennox iComfort systems, code 103 appears on the thermostat display; on older SureLight boards it flashes on the LED. The cause is almost always mechanical — blocked flue, clogged condensate, or a failed draft motor — not the switch itself.
Common Causes
- Blocked exhaust or intake vent — PVC vent pipes on 90%+ furnaces are vulnerable to bird nests, ice, and debris at the termination point. Any blockage prevents the draft motor from building sufficient pressure differential to close the switch.
- Condensate backup — High-efficiency Lennox furnaces produce condensate. When the collector box or drain trap clogs, water can flood the pressure switch port and prevent closure.
- Cracked or disconnected pressure switch hose — The rubber hose connecting the collector box to the switch is under vacuum and can crack at the fittings with age. Any leak means the switch reads zero pressure.
- Failed induced draft motor — A motor that hums but doesn’t spin (bad start capacitor) or spins slowly won’t build enough draft to close the switch.
Step-by-Step Fix
- Check vent terminations outside — Inspect both the exhaust and intake pipes at the exterior of the house. Clear any blockage. In cold climates, ice forming inside the exhaust pipe is a known issue — inspect from outside.
- Inspect the pressure switch hose — Trace the small rubber hose from the induced draft housing to the pressure switch. Look for cracks, kinks, or loose connections at either end. Replace if damaged.
- Clear the condensate system — Remove the condensate trap and flush it. Pour water into the collector box to verify it drains freely. On Lennox G61MP and SL280 series, the trap is at the bottom of the unit.
- Test the draft motor — With power on, the motor should spin within 5 seconds of a heat call. Listen for it. If it hums without spinning, pull the capacitor (disconnect power first) and test it.
- Reset the system — Power cycle after repairs. If code 103 clears and the furnace completes a full heating cycle, you’re done.
Parts Often Needed
| Part | Notes |
|---|---|
| Induced draft motor | Model-specific; Lennox uses several suppliers across product lines |
| Draft motor capacitor | Cheap fix if motor won’t start; test before replacing motor |
| Pressure switch | Match the WC rating printed on the old switch |
| Condensate trap | Replacement traps are model-specific on Lennox 90%+ units |
Technician Tips
Lennox furnace error codes are displayed as a series of LED flashes on the SureLight or integrated control board. A slow flash usually indicates normal operation, while rapid flashing points to a specific fault code. Code 1 = no previous code (normal standby). Code 2 = pressure switch failed open — check the vent, condensate drain, and inducer motor. Code 3 = pressure switch failed closed — replace the switch. Code 4 = high temperature limit — check airflow. Lennox systems with the SureLight igniter are particularly sensitive to gas pressure — verify 3.5” WC on natural gas before troubleshooting further. A 5-minute gas pressure check can save an hour of unnecessary component swapping.
Need Replacement Parts?
Finding the right replacement parts for your equipment is critical to getting it running again. Check current prices on Amazon — prices and availability change frequently.
When to Call a Pro
If the draft motor is running at speed, vents are clear, and condensate drains freely, but code 103 persists, a tech should measure static pressure with a manometer to determine whether the switch rating matches actual system pressure.
See Also
- Lennox Error Code 414 — Gas Valve Circuit Fault Fix
- Lennox Error Code 223 — Causes & Fix
- Lennox Harmony III Zoning System Error Codes — Complete Guide
- Lennox Error Code 414 Rollout — Causes & Fix