Lennox furnace Alert code 224: Low pressure switch stuck closed
Alert 224 means the low pressure switch is stuck closed. Lennox gives a specific threshold — closed for longer than 150 seconds during a heat call — which turns the diagnosis into a timed observation rather than a judgement.
Applies to: Lennox communicating gas furnaces on an S30, S40 or comparable communicating thermostat, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
The pressure switch proves the inducer is moving air before the control will admit gas. For that proof to mean anything the switch has to be capable of being open, and the control checks exactly that. A switch found closed when there is no suction cannot prove anything: it would read closed with a completely blocked flue just as readily. The 150 second figure is what makes this alert unusually actionable. Most manufacturers describe a stuck-closed switch qualitatively; Lennox states how long the control waits before deciding, which tells a technician what to watch and for how long.
Most common causes
- A failed pressure switch with stuck or welded contacts.
- A short in the pressure switch wiring, which is electrically identical to a closed switch.
- A switch jumpered during earlier service and never restored.
- A pressure switch of the wrong specification fitted in a previous repair.
- Moisture or corrosion in the switch or its connector bridging the contacts.
How to diagnose it
- Note whether the furnace has been serviced recently. A jumper left across a pressure switch is a real and dangerous cause.
- Lennox also directs a check of the operating pressure in inches of water column, and an inspection of the vent for blockage and the inducer for restriction.
- Nothing here is a homeowner check — it is the interlock that proves the flue is clear.
- Alert 223 is the opposite condition, the same switch failing to close.
How to fix it
- Technician work on a safety interlock.
- The wiring should be eliminated as a short before the switch is condemned.
- If a jumper is found it must come out, and the reason it was fitted has to be established.
- A replacement switch must match the specification for the model, not merely fit the fitting.
Notes
The seven-segment control reports this as E224, with the same meaning. Where Lennox's two code lists share a number they agree, which is not true of every manufacturer on this site.
Related
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.