Lennox furnace Alert code 271: Low pressure switch open — Watchguard

Alert 271 means the furnace has entered Watchguard mode after five failed attempts to prove draft on the low pressure switch during one demand.

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 is the escalated form of the low pressure switch alert. Rather than reporting a single failure to prove draft, it reports that the furnace tried five times within one heating demand and stopped. Lennox rates it Service Urgent, a step above the non-Watchguard version, which reflects that a repeated failure is a consistent condition rather than a transient one. The checklist is the same familiar set — vent blockage, inducer operation, cold end heater box and drain, cracked hoses, levelness — but the five strike count means a marginal or intermittent explanation is less likely than a real restriction.

Most common causes

• A blocked vent or intake at the outside termination.
• A blocked cold end heater box, condensate drain, or drain strainers.
• Cracked hoses in the pressure switch circuit.
• The unit not level, so condensate does not drain as designed.
• A weak or failing combustion air inducer.
• A pressure switch not closing at its rated pressure.
• Vent piping restricted or incorrectly sized.

How to diagnose it

1. Look at the outside vent termination first — highest yield and no tools needed.
2. Check the condensate drain for blockage.
3. Note that this alert means five consecutive failures, so a one-off explanation such as a gust of wind is unlikely.
4. Note the weather, since freezing at the termination produces exactly this.
5. Lennox references Service and Application Note H-13-07 for additional detail.

How to fix it

1. Clear the vent termination and condensate drain if either is blocked. Both are homeowner-reachable and both are on Lennox's list.
2. The pressure switch, hoses, inducer, and cold end heater box are technician work.
3. If the unit is not level, that needs correcting properly.
4. Never bypass a pressure switch. It is what prevents the furnace firing into a flue that cannot carry the products away.

Notes

The Watchguard alerts are worth distinguishing from their non-Watchguard counterparts. The same underlying fault reported after five strikes means something consistent is wrong, which narrows the plausible causes considerably.

Related

• Alert code 223 — Low pressure switch open
• Alert code 272 — Low pressure switch open in run mode — Watchguard

Sources

• Lennox Residential Communicating Systems Alert Code Guide, document 100017, March 2023 — Table 2, Alert Codes and Troubleshooting