Lennox furnace E274: Soft lockout — recycles exceeded, limit circuit opened
E274 means the furnace kept overheating and tripping its limit circuit until it exhausted its recycles, or the limit stayed open longer than three minutes. It is a one-hour soft lockout, and the cause is airflow.
Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, per installation instructions 508565-01.
What it means
This is the accumulated version of an overheat. A single limit trip is E250. E274 is what the control reports when limit trips have used up the recycle allowance, or when the limit remained open for more than three minutes. Lennox's action list is short and entirely about heat and air: shut down the system, check the firing rate, check the airflow, check for blockage. Nothing on that list is the limit switch itself, which is the point. A furnace reaching this code has overheated repeatedly, and the heat exchanger is the component that suffers from that.
Most common causes
- A dirty or clogged air filter — the most common cause by a wide margin.
- Closed, blocked or covered supply registers and return grilles.
- A restricted, undersized or crushed return duct.
- A blower wheel loaded with dirt, or a blower motor running slow or not at all.
- A gas firing rate set too high for the equipment.
- Blockage inside the heater.
How to diagnose it
- Check the air filter first and replace it if dirty. This is homeowner work and often the entire fix.
- Confirm registers and return grilles throughout the house are open and unobstructed.
- Note whether a very high MERV filter was fitted recently — a filter the system was not designed for restricts airflow.
- Note whether the furnace runs for a few minutes, quits, and goes quiet for about an hour.
How to fix it
- Replace a dirty filter and clear blocked registers before anything else.
- If it recurs with clean filters and open registers, the restriction is in the ductwork or the blower — technician work.
- Do not have the limit switch replaced to make the code stop. It is reporting a real overheat.
- Repeated overheating stresses the heat exchanger, and a failed heat exchanger is what puts combustion gases into the supply air.
Notes
Reaching E274 rather than a single E250 means this has been happening for a while. It is worth asking how long the furnace has been behaving this way before assuming it is recent.
Related
- E250 — Limit switch circuit open
- Alert code 274 — Primary limit switch open more than three minutes — Watchguard
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.