How do you fix Lennox 274?

Replace the air filter and open closed registers. Zone damper faults, firing rate adjustment, and gas pressure work are technician tasks. Full steps and safety notes are on this page.

Lennox furnace Alert code 274: Primary limit switch open more than three minutes — Watchguard

Alert 274 means the limit switch stayed open beyond three minutes and the system entered Watchguard mode. Lennox notes that in Smart Zoning systems this places the system into central mode.

Do the simple checks below, then stop. The simple checks listed above are genuinely homeowner work and worth doing first. Beyond them, this fault involves the gas valve, the burner compartment, or the heat exchanger — the parts whose failure can put combustion products, including carbon monoxide, into the air the house breathes. If the simple checks do not resolve it, stop there and call a licensed technician rather than resetting the furnace repeatedly to keep the heat on. Make sure there is a working carbon monoxide alarm on every level of the home, including the level the furnace is on. The conditions that mean stop now.

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 primary limit alert, triggered when the switch remains open past three minutes. Lennox rates it Service Urgent, a step above the non-escalated version, which is appropriate — a limit that opens briefly may be a marginal condition, while one that stays open for minutes means the furnace is substantially overheating. The zoning note matters practically: on a Lennox Smart Zoning system, a limit trip of this kind puts the whole system into central mode, so the symptom a householder notices may be that zoning has stopped working rather than that the furnace faulted.

Most common causes

High gas pressure, which Lennox lists first.
Low supply air from a plugged filter or a blockage in the duct work.
An incorrect firing rate on the furnace.
Non-functioning zone dampers restricting airflow.
Closed or blocked supply registers.
A failing blower motor or a blower wheel packed with dust.
Undersized or restrictive ductwork.

How to diagnose it

Check the air filter first — Lennox gives a dirty filter as its example of restricted supply air.
Walk the house and check registers and the return grille.
If the house has zoning, note whether it has reverted to central mode, which Lennox documents as a consequence of this alert.
Check whether zone dampers appear to be operating, since Lennox lists non-functioning dampers as a cause.
Lennox references Service and Application Note ACC-14-01 for corrective actions.

How to fix it

Replace the air filter and open closed registers.
Zone damper faults, firing rate adjustment, and gas pressure work are technician tasks.
Do not keep resetting the furnace. Sustained overheating is what cracks heat exchangers, and this alert means the overheat lasted minutes rather than seconds.
Have the zoning system checked as well as the furnace, since a stuck damper causes this without any filter or duct being obviously wrong.

Notes

Zoning is an easily overlooked cause of overheating. A damper that has failed closed restricts airflow through the furnace in a way that no amount of filter changing will fix, and the householder's experience is simply that some rooms are cold.

All codes for this manufacturer

Lennox furnace alert codes — every code, with what differs between models.

What the fix costs

The parts math is the standard limit-circuit band from the repair guide — $150–350, find why it tripped — but the three-minutes-open threshold changes the risk side of the ledger. A limit that blips open and recloses caught a marginal overheat; one that stays open past three minutes was holding back a furnace running seriously hot, and every additional lockout-and-retry cycle re-bakes the heat exchanger. Since heat exchanger failure ($1,000–2,500+, usually the end of the furnace) is the downstream cost being risked, the Service Urgent rating translates financially as: the $75–150 diagnostic today is protecting the four-figure component. On zoned systems, add the possibility that the finding is a stuck damper in the zoning hardware rather than anything in the furnace cabinet.

DIY or call a pro

Check and change the filter — the one universal homeowner move on any overheat alert — and open closed registers, but on 274 specifically, treat that as your only turn at bat. If the alert returns with airflow visibly unrestricted, do not settle into the furnace's hourly retry rhythm and wait for the weekend: sustained-open limit trips are the overheat variant that does cumulative damage, and Lennox's urgency rating exists for it. Watch for the zoning tell while you wait for the visit — on a Smart Zoning system this alert forces central mode, so every room getting the same air is your confirmation that a trip occurred even if you never saw the thermostat alert. Report that, the filter's condition when you found it, and whether any zone dampers have been audibly clunking or sticking. The firing-rate and duct-static measurements that close out the stubborn cases are instrument work, and a technician doing them is also positioned to inspect for the heat stress this alert implies.

What this looks like

From the outside, this usually shows up as the furnace heats then trips out on high limit — that walkthrough covers what to check in order, and when it stops being a homeowner job.

Sources

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

This site is a reference, not a substitute for a licensed HVAC technician. Gas appliances can leak carbon monoxide and cause fire. Anything involving gas lines, the heat exchanger, or the flame sensing circuit should be handled by a licensed professional. Pages are written with AI assistance from the sources listed on each page and checked against them before publishing.