Lennox furnace Alert code 250: Primary limit switch open

Alert 250 means the primary limit switch is open. Lennox documents that if it does not close within three minutes the unit enters a 60 minute soft lockout, called Watchguard mode.

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

The primary limit opens when the furnace overheats, and Lennox's documentation gives the exact escalation: three minutes open, then a 60 minute soft lockout. That timing produces the familiar pattern of a furnace that heats briefly roughly once an hour. Lennox's cause list is airflow-focused as expected — low supply air from a dirty filter or duct blockage — but it adds two items other manufacturers do not: checking the furnace's firing rate, and checking for non-functioning zone dampers. It also notes that a limit trip places a Lennox Smart Zoning system into non-zone mode, which may require manual reactivation.

Most common causes

• Low supply air due to a dirty air filter, which Lennox names as an example.
• Blockage in the duct work.
• High gas pressure, which Lennox lists first.
• An incorrect firing rate on the furnace.
• Non-functioning zone dampers, which Lennox names specifically.
• Closed or blocked supply registers.
• A failing blower or a blower wheel packed with dust.

How to diagnose it

1. Check the air filter first, which Lennox gives as its example of restricted supply air.
2. Note the hourly pattern if present, which matches the documented 60 minute Watchguard behaviour.
3. Walk the house and check registers and the return grille.
4. If the house has zoning, note whether dampers appear to be operating — Lennox lists non-functioning dampers as a cause and notes zoning goes to non-zone mode after a limit trip.
5. Lennox references Service and Application Note ACC-14-01 for further detail.

How to fix it

1. Replace the air filter and open closed registers.
2. Zone damper faults, firing rate adjustment, and gas pressure are technician work.
3. If the system uses Lennox Smart Zoning, note that zoning may need manually reactivating after a limit trip — Lennox states the field will need to do this.
4. Do not keep letting the furnace lock out and restart hourly. Each cycle is another overheating event, and repeated overheating is what cracks heat exchangers.

Notes

The zoning interaction is worth knowing about. A zone damper that has failed closed reduces airflow through the furnace without any filter or duct being obviously wrong, and it is not something a homeowner would normally think to check. This is alert code 250. There is no alert code 251 in the Lennox guide, despite it appearing in some third-party lists.

Related

• Alert code 274 — Primary limit switch open more than three minutes — Watchguard
• Alert code 252 — Discharge air temperature high

All codes for this manufacturer

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

What the fix costs

The repair guide prices limit switch work at $150–350 with the pointed note that the job is finding why it tripped, not just the part — and on this alert the why is usually airflow, where the fixes are the cheapest on the site: a filter, opened registers, or (Lennox-specifically) a zone damper that stopped moving. The switch itself is rarely the failure. The bill that matters is the one avoided: a furnace that trips its limit every hour is heat-stressing its heat exchanger on every cycle, and heat exchangers at $1,000–2,500+ are the repair that usually ends a furnace. Fixing a $20 airflow problem promptly is the best return on money anywhere in this code list.

DIY or call a pro

Start with the one check that resolves more 250s than everything else combined: the filter. If it is loaded, replace it, confirm supply registers and returns are open, and let the furnace try again — that much is squarely homeowner work and directly on Lennox's cause list. The escalation logic is what to respect: if the alert returns with a clean filter and open vents, stop cycling it through its one-hour lockouts to harvest heat, because each trip re-runs the overheat. The remaining causes are professional by nature — firing rate is a combustion measurement, duct restriction is an airflow measurement, and on a Smart Zoning system a stuck damper diagnosis runs through the zoning controls. Mention the zoning system when you book if you have one; Lennox notes a limit trip drops it into non-zone mode, which itself may need manual reactivation.

The same fault on other brands

An open high limit is coded in every family this site covers — the limit switch hub tracks 17 versions across 11 families, with the recurring editorial point that the cause is almost never the switch. Carrier's code 33 (which folds rollout into the same circuit), Goodman's 4 flashes, and Rheem's 3 blinks are direct equivalents. What Lennox adds that the others do not: the three-minute threshold before the one-hour lockout, and the zone-damper and firing-rate items on its cause list.

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.

This can also show up as a burning or gas smell from the unit — that page sorts the normal smell from the ones that mean shut it down.

Sources

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