Lennox furnace Alert code 228: Inducer calibration issue
Alert 228 means the furnace control could not calibrate the pressure switch. The furnace performs a calibration routine, and this alert says it did not complete.
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
Lennox's communicating furnaces run a pressure switch calibration in which the control establishes the relationship between inducer speed and the pressure the switch sees. This alert means that routine failed. It is a distinctly modern fault — older furnaces simply had a switch with a fixed setpoint and no calibration step at all. The value of the calibration is that it lets the control adapt to a specific installation's venting; the cost is that anything disturbing the pressure signal prevents it completing. Lennox's checklist is the familiar one: vent blockage, inducer restriction, cold end heater box and drain, cracked hoses, and levelness.
Most common causes
- A vent blockage or restriction preventing a stable pressure reading.
- The combustion air inducer not operating correctly, or restricted.
- A blocked cold end heater box, condensate drain, or drain strainers.
- Cracked hoses in the pressure switch circuit.
- The unit not level.
- A pressure switch that has drifted out of its expected range.
How to diagnose it
- Check the outside vent termination for obstruction.
- Check the condensate drain for blockage or slow drainage.
- Note whether the furnace is level, since Lennox lists it among the checks.
- Note whether this appeared after installation or after service, since calibration runs at those points.
- Note that Lennox says this clears automatically after a successful calibration, so the alert going away means calibration succeeded rather than that a cause was found.
How to fix it
- Clear the vent termination and condensate drain if either is restricted. Both are homeowner-reachable.
- The inducer, pressure switch, hoses, and cold end heater box are technician work.
- If the unit is not level, correcting that properly matters for condensate drainage generally, not only for this alert.
- Lennox references Service and Application Note H-13-07 on condensing furnace pressure switch troubleshooting — worth a technician consulting.
Notes
Calibration-based pressure proving is more adaptive than a fixed switch and gives the control better information about the installation. It also means a furnace can fault for reasons that would not have troubled an older design, which is a real trade-off rather than a defect.
Related
- Alert code 226 — High pressure switch stuck closed
- Alert code 229 — Ignition on high fire — information only
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
A failed calibration is a fault in a process, not necessarily in a part, and the pricing follows that logic. Most 228s resolve inside a diagnostic visit ($75–150) once whatever was disturbing the pressure signal — a restricted vent, a backed-up drain, a cracked hose — is corrected and the routine is allowed to complete. The parts outcomes mirror the other pressure alerts: switch work at $150–400, or $300–900 where the inducer itself cannot produce a clean, repeatable pressure curve for the control to calibrate against. One 228-specific caution for the quote: because calibration is the control “learning” this installation's venting, a failure right after vent modifications or a new install points at the venting design, and correcting that is the installer's conversation, not a parts bill.
DIY or call a pro
The calibration routine needs a stable pressure environment to succeed, and the homeowner checks are about removing the obvious instabilities: terminations clear on both pipes, condensate moving, nothing leaning against or capping the vent outside. If the alert appeared in wild weather, letting the system retry in calm conditions is legitimate — a calibration disturbed by a storm is not a broken furnace. Where DIY ends is where the diagnosis becomes a comparison between commanded inducer speed and measured pressure, which is exactly the technician instrumentation this alert exists to summon. Worth stating because this is a calibration fault: do not power-cycle the furnace repeatedly to force re-runs. If two attempts in decent conditions fail, the signal path has a real problem, and each additional forced attempt just re-documents it.
What this looks like
From the outside, this usually shows up as the inducer spins up but the burners never fire — that walkthrough covers what to check in order, and when it stops being a homeowner job.