Lennox Error Code 225 — What It Means
Lennox fault code 225 is a pressure switch fault — the board commanded the inducer on and waited for the pressure switch to confirm adequate draft, but the switch did not close within the allowed window. On high-efficiency (90%+) Lennox units, there are often two pressure switches (low-fire and high-fire), and code 225 typically applies to the primary draft proving switch. The furnace will not proceed to ignition and will retry a defined number of times before locking out. This fault is extremely common in winter when condensate freeze and flue blockages peak.
Common Causes
- Waterlogged pressure switch hose — Condensate from the flue gases can migrate back up the small rubber hose connecting the inducer housing to the pressure switch, partially or fully blocking airflow through the hose and preventing the diaphragm from deflecting.
- Blocked or frozen flue termination — The PVC vent cap on the side or top of the building becomes blocked with ice, snow, or debris, creating back pressure that prevents the inducer from developing adequate draft.
- Failed pressure switch — The pressure switch diaphragm or contacts fail and the switch never closes regardless of actual draft pressure. Test with a manometer or by jump-testing (temporarily) under supervised conditions.
- Cracked or disconnected inducer housing — A crack in the inducer housing or a gasket leak allows the pressure differential to equalize, and the switch never sees sufficient negative pressure.
Step-by-Step Fix
- Check the pressure switch hose for water — Disconnect the hose at both ends (inducer housing port and pressure switch inlet). Tip it toward a rag and look for water. If present, blow the hose clear, then check the condensate drain for blockage.
- Inspect the flue termination — Go outside and verify the PVC vent cap is clear. Remove any ice buildup, snow packing, or debris blocking the outlet opening. Check both the exhaust and combustion air intake pipes.
- Test pressure switch with a manometer — Connect a digital manometer to the inducer housing pressure port. With the inducer running, measure the negative pressure produced. Compare against the pressure switch setpoint (stamped on the switch body).
- Test the pressure switch directly — Use a multimeter to check switch continuity. At rest (no draft) it should be open. Apply mouth suction to the inlet port — a good switch closes at the rated pressure. Replace if it fails this test.
- Inspect inducer housing and gaskets — With the unit off, inspect the seam where the inducer mounts to the furnace. Any gaps, cracked plastic, or deteriorated gaskets allow pressure equalization and cause this fault.
Parts Often Needed
| Part | Notes |
|---|---|
| Pressure switch | Match exact setpoint — Lennox 90%+ units commonly use −0.6” or −1.0” W.C. |
| Pressure switch hose | Cheap; replace any hose that’s cracked, brittle, or clogged |
| Inducer housing gasket | Required if housing is leaking at the mounting interface |
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 flue is clear, the pressure switch hose is dry, and a new pressure switch still doesn’t close with the inducer running, the inducer is likely not producing adequate draft — possibly due to a failed motor, cracked wheel, or significantly undersized flue run. This requires hands-on measurement and may require a venting redesign.