Lennox furnace E271: Soft lockout — retries exceeded, pressure switch opened
E271 means the furnace exhausted its ignition retries and the last one failed specifically because the pressure switch opened. The furnace is not failing to light — it is losing proof of draft.
Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, per installation instructions 508565-01.
What it means
This code is doing something most furnace controls do not: recording why the final attempt failed, not merely that it did. The retries were exhausted, and the last one ended with the pressure switch opening rather than with a failure to prove flame. That points the repair at the draft side — vent, inducer, condensate, hoses — rather than at the ignitor, gas valve or flame sensor, which is where a generic ignition lockout would send someone. Lennox's steps are the pressure measurements: the pressure in inches of water column at which the switch closes on a heat call, the actual operating pressure, and an inspection of the vent and combustion air inducer for restriction.
Most common causes
- A partially restricted vent or intake, enough to start but not to sustain.
- Condensate accumulating during the cycle in the collector box, hose or drain.
- An inducer weakening under load from bearing wear or a dirty wheel.
- A pressure switch operating right at its setpoint.
- A cracked or kinked pressure switch hose.
How to diagnose it
- Check the vent and intake terminations outside for partial obstruction as well as complete blockage.
- Note whether the fault is worse in wind or in very cold weather.
- Note any standing water near the furnace.
- Note how far into the sequence the furnace gets before quitting — the timing helps a technician.
How to fix it
- Clear any obstruction at the terminations.
- The measurements Lennox specifies are technician work and are what separate a weak inducer from a marginal switch.
- Do not jumper the pressure switch. It is the interlock proving the flue is clear.
- The lockout clears when a heat call finishes successfully.
Notes
E271 and E272 both end at the pressure switch but arrive differently — E271 through failed retries during ignition, E272 through recycles after the furnace was already running.
Related
- E272 — Soft lockout — recycles exceeded, pressure switch opened
- E227 — Pressure switch opened during ignition or run
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
The money value of E271 is in what it rules out. A furnace that locks out during ignition attempts invites spending on ignition parts — and this code's second clause, “last retry failed due to the pressure switch opening,” exonerates the ignitor, the flame sensor and the gas valve before a dollar moves. The eligible spending is all draft-side, per the repair guide: pressure switch work at $150–400 (blocked flue checked first, per the guide's own note), or the inducer at $300–900 when the measurements show it cannot hold its pressure through the ignition sequence. If a quote for this code leads with an ignitor or sensor replacement, the code has not been read — which is worth knowing before approving anything, since the control documented its reason for stopping. More codes like this across brands: pressure switch codes.
DIY or call a pro
What is distinctive here is that the pressure switch closed and then let go during the trial for ignition, attempt after attempt — a draft that starts and cannot hold even briefly. The homeowner checks are the standard draft set, but with an eye for the marginal rather than the blocked: terminations that are partially crusted with ice or rime (common in the exact cold snaps when the furnace works hardest), and a condensate path that is slow rather than stopped. Clear what you find, then allow the furnace its own retry when the soft lockout expires rather than forcing resets — the clearing condition is a successfully completed heat call, and it will attempt one unprompted. The measurements that decide between a fading inducer and a marginal switch are manometer work on a live sequence, and the switch itself stays un-jumpered under every circumstance; it is the only witness to whether the flue can support ignition at all.
What this looks like
From the outside, this usually shows up as the furnace turns over but never lights — that walkthrough covers what to check in order, and when it stops being a homeowner job.