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.
What the fix costs
By the repair cost guide's ranges this lands at $150–400 if the switch itself has welded closed — but stuck-closed has two cheaper endings worth knowing about before a part is quoted. A short in the switch wiring reads electrically identical to closed contacts and costs a wiring repair, not a switch; and a jumper left from a previous service call costs nothing to remove, though the visit must then answer why someone fitted it — a jumper usually means an earlier technician bypassed a real draft problem instead of fixing it, and that original problem is still there, now with its alarm silenced. Whatever the finding, a replacement switch must match the model's pressure specification, which is why this is not a parts-counter purchase.
DIY or call a pro
This alert is the firmest call-a-pro on the Lennox pressure switch pages, and the reason is what stuck-closed means: with the switch asserting draft permanently, the control would accept a completely blocked flue as proven-clear. Every other pressure switch alert fails safe — this one fails silent. There is no vent termination to clear or drain to flush that produces this reading, so there is no homeowner fix to attempt; the useful thing you hold is the service history. Before the visit, recall whether the furnace was worked on recently and by whom, because a jumpered switch or a wrong-spec replacement from that visit is a leading cause, and saying so points the diagnosis in one sentence. Then let the technician run Lennox's own sequence — wiring first, then the switch against its rated pressure, then the 150-second observation.
The same fault on other brands
The defeated-interlock reading — switch closed before the inducer has even started — gets its own code almost everywhere: Carrier's code 23 (did not open), Goodman's 2 flashes on the White-Rodgers-lineage boards, and Rheem's *RC-board 3 blinks all describe the same condition, and the pressure switch hub maps the full family. Lennox's 150-second threshold is the unusual part — most brands report the state, Lennox reports how long the control watched it before deciding.
What this looks like
From the outside, this usually shows up as the furnace starts and stops in short bursts — that walkthrough covers what to check in order, and when it stops being a homeowner job.