How do you fix Lennox 252?

Replace the air filter with the correct size and type and open closed registers. This addresses the most common cause and is homeowner work. Temperature rise measurement, airflow adjustment, and input rate correction are technician tasks. Full steps and safety notes are on this page.

Lennox furnace Alert code 252: Discharge air temperature high

Alert 252 means the air leaving the furnace is too hot. It is an early warning of the condition that eventually opens the limit switch, caught before the switch has to act.

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

Lennox's communicating furnaces measure discharge air temperature directly, which lets the control notice overheating developing rather than waiting for a limit switch to open. That makes this alert more useful than the limit switch alert it precedes: the furnace is still running, the heat exchanger has not yet reached the point where the safety must act, and the underlying cause is the same. Lennox's guidance names three things to check — temperature rise, airflow, and input rate — plus dirty air filters. Treated as a prompt, this alert prevents the limit trips that damage heat exchangers over time.

Most common causes

Dirty air filters, which Lennox names explicitly.
Airflow lower than the furnace requires.
An input rate set too high for the airflow available.
Temperature rise outside the range on the furnace's rating plate.
Restrictive or undersized ductwork.
Closed or blocked registers.
Zone dampers reducing airflow through the furnace.

How to diagnose it

Check the air filter, which is Lennox's first named cause and the most common.
Walk the house and check registers and the return grille.
Note whether the alert appears on long runs, which fits a marginal airflow condition.
Note whether limit switch alerts have also appeared, since this alert normally precedes them.
Lennox references Service and Application Note ACC-14-01 for further detail.

How to fix it

Replace the air filter with the correct size and type and open closed registers. This addresses the most common cause and is homeowner work.
Temperature rise measurement, airflow adjustment, and input rate correction are technician tasks.
Act on this alert while it is only a warning. It is reporting the condition that eventually cracks heat exchangers, before the safety has had to intervene.
If it recurs after a filter change, have the ductwork and airflow assessed rather than continuing to change filters.

Notes

This is one of the genuine advantages of a communicating system. A conventional furnace has no way to report that it is running hot but not yet unsafe — it simply runs hot until the limit opens, and every one of those trips is wear on the heat exchanger.

All codes for this manufacturer

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

What the fix costs

Caught at this stage, the fix is usually the cheapest item in the airflow chain: a filter, a closed-register audit, or at most a service visit ($75–150) to measure temperature rise and check the input rate — the two Lennox-named checks that need instruments. Compare that against the path this alert exists to interrupt: sustained overheating matures into limit trips at $150–350 a diagnosis, and chronic limit-trip operation is how heat exchangers accumulate the thermal stress that ends in the $1,000–2,500+ replace-the-furnace conversation. Alert 252 is the discount window — the same underlying fault at its minimum possible price, offered before any safety has had to act.

DIY or call a pro

This is the most homeowner-favorable alert in the Lennox heating list, because it fires while everything still works: the furnace is running, nothing is locked out, and the most probable cause is the filter you can hold in your hand. Change it if it is dirty, walk the house opening any closed supply registers, and make sure returns are not blocked by furniture — then watch whether the alert recurs. Where it stops being yours: temperature rise is measured against the range on the furnace's rating plate with thermometers in the duct, and input rate is a gas-side measurement — both technician work, and both worth doing if the alert persists with clean, open airflow, because a furnace overfiring for its ductwork is a real and fixable finding. The mistake to avoid is the quiet one: dismissing a warning that arrives while the heat still works, which is precisely when the repair is cheapest.

What this looks like

From the outside, this usually shows up as there is no heat but nothing looks obviously wrong — 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.