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.
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 241 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
Lennox alert code 250 on a communicating system reports the same condition — an open primary limit. The two Lennox lists agree here. An earlier version of this page said they conflicted, which was wrong.
Related
- Alert code 250 — Primary limit switch open
- E274 — Soft lockout — recycles exceeded, limit circuit opened
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
Read the price list in the order Lennox lists the causes and it descends from instrument work to pocket change: firing rate is a combustion measurement (technician, part of a $75–150 diagnostic), blockage-in-heater and airflow checks span from duct problems down to the humble filter, and the switch itself — at $150–350 with the guide's find-why-it-tripped warning — is the last suspect, not the first. The rhythm is the cost signal to take seriously: the three-minute/one-hour logic means a furnace with a persistent airflow problem delivers one short burn per hour, which households often endure for weeks as “the heat being weird.” Every one of those hourly cycles is an overheat event, and the component absorbing them is the heat exchanger, whose $1,000–2,500+ replacement is the bill this code is trying to spare you.
DIY or call a pro
One homeowner intervention is legitimate and often decisive: a genuinely clean filter and every register and return open, done once, immediately. What matters as much on E250 is recognizing the pattern that means your intervention did not work — heat for a few minutes, silence for an hour, repeat — and declining to live with it. The blower running steadily through the quiet periods is the furnace deliberately clearing heat, not a second fault, so resist the urge to power-cycle out of what looks like a stuck state; the control is mid-protocol. If the hourly rhythm survives a filter change, the remaining causes (firing rate, internal blockage, duct restriction) all need instruments, and the visit should be booked rather than the lockouts endured. Tell the technician how many days the pattern ran; it informs how hard they look at the heat exchanger.
The same fault on other brands
Among seven-segment-display peers, Goodman's E3 and Trane's E04 report the same open-limit condition, and the legacy York 4 red flashes matches on the JCI boards — the limit switch hub holds the full seventeen-code family and its consistent moral that the switch is the messenger. Lennox's published three-minutes-then-one-hour escalation is the detail its peers leave undocumented, and it is what makes the once-an-hour symptom identifiable from the couch.
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.