Quick answer
Lennox furnace error code E250 means the limit switch circuit is open: the furnace overheated. The burners stay off while the limit is open, and if it has not closed after 3 minutes the furnace goes into a 1-hour soft lockout. Replace the filter and open all supply and return registers. If E250 returns with a clean filter, firing rate, temperature rise and airflow need measuring.
The short version of what Lennox furnace error code E250 means: the limit switch circuit is open because the furnace overheated. The limit is a temperature safety, and it opens when the furnace gets hotter than its design allows. Lennox lists E250 this way in the diagnostic code table for the EL296DFV. Confirm against the label inside your own furnace panel, since tables differ by model.
What the furnace does while the limit is open
The burners stay off for as long as the limit is open. If the switch has not closed again after 3 minutes, the control puts the furnace into a 1-hour soft lockout before it will try to heat. So E250 can look like a furnace that heats for a while, quits, and comes back an hour later.
That behaviour is the furnace protecting its heat exchanger, and it is safe in the moment. It is not something to live with. Repeated overheating is hard on the heat exchanger, and a limit that keeps opening is reporting a real airflow or firing problem.
Replace the filter and open every register first
Low airflow is the leading cause, and it is the part a homeowner can fix. Do both steps before deciding the furnace is faulty.
- Pull the filter and hold it up to a light. If you cannot see light through it, replace it with the same size, arrow pointing toward the furnace.
- If the filter is a dense pleated type in a 1-inch slot, try a less restrictive one. Some duct systems cannot pull enough air through them.
- Walk the house and open every supply register, including the ones in unused rooms and behind furniture.
- Find the return grilles, which are usually larger, and move rugs, boxes and furniture away from them.
- Let the lockout time out, then run a heat cycle and see whether E250 stays away.
How often to change a furnace filter has the schedule by filter type, and whether to close vents in unused rooms explains why closed registers push a furnace toward its limit.
Causes of E250, most likely first
- Low airflow from a dirty filter, closed registers or a duct restriction such as a crushed run or a dust-matted air conditioning coil above the furnace.
- A blower problem: a wheel caked with dust, a failing motor, or a blower set to the wrong speed for the furnace.
- An over-fired furnace, where the burners put out more heat than the rating plate allows.
- A blockage in the heat exchanger section that stops air passing evenly over the hot surfaces.
Duct restriction is a known pattern in 1950s and 60s bungalows across Burlington, Hamilton and Etobicoke, where the original return ducts were sized for a different furnace. A modern furnace on undersized returns runs hot even with a clean filter, and E250 is how it says so.
Skip the wait-and-see approach in two situations. If the burners light and no air ever comes out of the registers, the blower is not running and the limit will open on every cycle. If E250 comes back on the first cycle after a new filter, the restriction is somewhere you cannot reach.
What a technician measures when E250 returns with a clean filter
Three numbers settle it: firing rate, temperature rise and airflow. The technician checks the gas input against the rating plate, measures the air temperature entering and leaving the furnace to see whether the rise falls inside the range printed on that plate, and reads duct static pressure to find where the air is being held back.
The limit switch itself is tested last. A limit that opens on a furnace running inside its rated temperature rise is faulty; one that opens on a furnace running hot is doing its job.
- Service call with diagnosis: $90–$150, credited toward the repair
- Furnace tune-up, including a blower and burner check: $120–$250
- Blower motor replacement: $500–$1,500 depending on motor type
- Limit switch replacement or duct corrections: quoted in writing after the measurements, since the part and the labour vary by furnace
When a new filter and open registers do not clear E250, the cause is measurable and fixable. ZK Mechanical handles furnace repair for Lennox equipment in Burlington, Oakville, Milton, Hamilton, Mississauga and Brampton, and quotes the fix before starting. Send us the code and your furnace model, or look up other codes in the Lennox furnace error code guide.