Lennox furnace Alert code 272: Low pressure switch open in run mode — Watchguard
Alert 272 means the low pressure switch opened during a run and the system entered Watchguard mode. Lennox rates it Service Urgent.
Applies to: Lennox communicating gas furnaces on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
This alert combines two things worth separating: the fault occurred while the furnace was already running, and it was serious enough to trigger Watchguard. The run-mode timing means draft was adequate at ignition and was lost during the cycle, which points at conditions that develop as the furnace operates. Lennox's specific question here is whether the switch is stuck open during a heat call, and its checklist covers vent blockages, inducer operation and restriction, the cold end heater box and drain, cracked hoses, and levelness.
Most common causes
- A condensate drain or cold end heater box backing up during a run.
- Ice forming progressively at the exhaust termination during a long cycle.
- A pressure switch stuck open during the heat call.
- An inducer weakening as it warms.
- Cracked hoses in the pressure switch circuit.
- A partly restricted vent where draft is marginal.
- The unit not level, so condensate accumulates during operation.
How to diagnose it
- Note that the furnace lit successfully before faulting — that distinguishes this from a failure to prove draft at all.
- Note how long the furnace runs before faulting, since a consistent interval points at something accumulating.
- Check the condensate drain for slow drainage.
- Check the outside vent termination, and note the weather.
- Check whether the furnace is level.
How to fix it
- Clear a partly blocked condensate drain and the vent termination — both homeowner work.
- The pressure switch, hoses, inducer, and cold end heater box are technician tasks.
- Tell the technician how long the furnace runs before faulting. That observation distinguishes several causes and only the householder can provide it.
- Never bypass a pressure switch.
Notes
Run-mode faults are harder to reproduce on a service visit than start-up faults, because the technician has to wait out a full cycle. Knowing the elapsed time in advance saves that wait and makes the visit more likely to be productive.
Related
- Alert code 227 — Low pressure switch open in run mode
- Alert code 271 — Low pressure switch open — Watchguard
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
Why run-mode plus Watchguard earns Service Urgent
Each half of this alert's name downgrades the benign explanations. Watchguard means five strikes in one demand, so a single gust of wind or a momentary drain gurgle is off the table — whatever opens the switch does it every cycle. Run mode means the furnace was burning when draft proof was lost, and that is the half that justifies the urgency rating: a start-up failure leaves a cold furnace waiting politely, but a run-mode failure means combustion was underway while the flue's measured capacity was collapsing. The control did its job and shut things down — five times — and the alert is the record of how consistently it had to. Treat the Service Urgent label as Lennox's way of saying this fault has stopped being intermittent.
What the fix costs
The repeatability that makes this alert urgent also makes it cheap to diagnose: a fault that appears every cycle, a known number of minutes in, can be caught live on a single diagnostic visit ($75–150, often credited against the repair) instead of the two or three visits an intermittent fault burns. From there the repair guide's ranges apply: pressure switch work at $150–400 — with the guide's blocked-flue-first caveat — or $300–900 if the real problem is an inducer that fades once hot. The evidence that keeps it to one visit is yours to collect: how many minutes the furnace runs before it quits, every cycle, for a few cycles.
DIY or call a pro
Two of Lennox's listed causes are homeowner-reachable and worth clearing before the call: a condensate drain that runs slow (watch it while the furnace runs — this fault develops mid-cycle, so a static look can miss it) and the outside termination, where ice built during a long cold-weather run produces exactly this signature. The stopwatch work described above is the third homeowner job, and on a run-mode fault it is genuinely diagnostic. Beyond that, stop: the switch, its hoses, the cold end heater box and the inducer are all technician territory, and bypassing the switch to keep heat on is the one move this page exists to talk you out of — on this alert the furnace is already demonstrating, five times per demand, that its flue cannot sustain the burn.
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.