Lennox furnace E250: Limit switch circuit open
E250 means the limit switch circuit is open — the furnace is overheating. If the limit does not close within three minutes, the furnace goes into a one-hour soft lockout.
Applies to: Lennox gas furnaces with a seven-segment diagnostic display on the control board, such as the ML193UHEK, per installation instructions 508565-01. On Lennox's communicating alert code list, 250 means something completely different.
What it means
The limit circuit is a string of normally-closed temperature switches that open when the air leaving the furnace gets too hot. Opening the circuit cuts the gas while leaving the blower running to clear the heat. Lennox specifies the timing precisely: if the limit has not closed within three minutes, the unit drops into a one-hour soft lockout, which is why a furnace with this fault often appears to work for one cycle and then go quiet for an hour. The cause is almost never the limit switch. Lennox's own steps name the firing rate, blockage in the heater, and airflow — all of which are the furnace producing more heat than the air can carry away.
Most common causes
- A dirty or clogged air filter, which is the most common single cause.
- 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.
- A gas firing rate set too high for the equipment.
- Blockage inside the heater itself.
How to diagnose it
- Check the air filter first and replace it if dirty. Homeowner work, and often the whole repair.
- Walk the house and confirm registers and returns are open, including under furniture.
- Note whether the furnace runs for a few minutes and then stops with the blower still going — that is this fault's signature.
- Confirm the code came from the display on the control board, not the thermostat. Alert code 250 on a communicating system is flame sensed out of sequence, an unrelated and more urgent fault.
How to fix it
- Replace a dirty filter and clear blocked registers, then see whether it returns.
- If it returns with clean filters and open registers, the restriction is in the ductwork or the blower — technician work.
- Replacing a limit switch that is doing its job leaves the overheat in place and removes the protection against it.
- Repeated limit trips stress the heat exchanger, which is the part whose failure lets combustion gases into the supply air.
Notes
This is the specific code where Lennox's two systems disagree, and the disagreement matters: airflow problem on one list, gas valve concern on the other.
Related
- Which Lennox code list applies — Lennox uses two code systems that disagree on code 250
- Alert code 251 — Primary limit switch open
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.