Lennox furnace E240: Low flame current in run mode
E240 means the flame is lit but the current the control reads back from the flame sensor is too low. It is the code that precedes a furnace that starts shutting down mid-cycle.
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
Flame sensing works by rectification: the control puts AC on a rod sitting in the flame, and the ionised gas conducts in one direction only, returning a few microamps of DC. E240 says that current is present but weak. The most useful thing about this furnace is that it will tell you the number — the seven-segment display has a flame signal mode, selected with the push button, that shows the flame current in microamps directly. That turns a judgement into a reading, and it means a technician can confirm the problem and confirm the repair rather than cleaning the sensor and hoping. Lennox's own steps are to check the microamps using the control diagnostics, clean or replace the sensor, and measure the voltage from neutral to ground to confirm the unit is properly grounded.
Most common causes
- A flame sensor coated in oxide, which builds up normally over a heating season and is not visible as dirt.
- A poor chassis ground, since the rectification circuit is referenced to it.
- Reversed line voltage polarity.
- A sensor cracked, or positioned so the flame does not fully envelop it.
- Burners partially blocked, so the flame is not reaching the rod properly.
How to diagnose it
- Note whether the furnace has begun shutting down part way through a cycle, which is where this ends up if it is left.
- Note the age of the furnace and when the flame sensor was last cleaned — this is routine maintenance, not a defect.
- Ask a technician to read the flame current from the control's flame signal mode before and after cleaning. This furnace makes that easy.
- Do not attempt to clean the sensor yourself. It is in the burner compartment.
How to fix it
- Technician work in the burner compartment.
- Cleaning or replacing the flame sensor is the common repair, and the microamp reading confirms whether it worked.
- Ground and line polarity should be checked before the sensor is blamed, since both produce a low reading with a perfectly good sensor.
- Lennox notes the alert clears once the current heat call completes.
Notes
The same code number, 240, exists on Lennox's communicating alert list with the same meaning. This is one of the codes where the two systems agree.
Related
- Alert code 240 — Low flame current in run mode
- E273 — Soft lockout — recycles exceeded after flame failure
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.
What the fix costs
The likely invoice is the repair guide's cheapest real repair — flame sensor cleaning at $80–200 — but this board is one of the few that lets you buy that repair with proof attached. Because the seven-segment display's flame signal mode reads out microamps directly, a thorough technician can show you the number before the cleaning and after it, converting a take-my-word-for-it service into a measured one. Worth requesting explicitly. The 508565-01 action list for E240 also directs measuring neutral-to-ground voltage to verify the unit's ground, which is the difference between a cleaning that holds for seasons and one that fades in weeks: if the ground is marginal, the sensor was never the problem, the fix is electrical, and the cost stays in service-call territory rather than becoming a quarterly cleaning habit.
DIY or call a pro
The sensor lives at the burner assembly and this site keeps the flame-sensing circuit on the professional side of the line, so the wipe-the-rod job — however often the internet recommends it — is not what this page suggests. The two homeowner moves that actually bear on a weak flame signal sit outside the cabinet: keep the filter honest, since disturbed burner airflow shows up in flame quality, and note the code's timing, because E240 is specifically a run-mode code — the flame lights and holds, and the signal sags while burning, which is a different story from a furnace that never proves flame at all (that pattern belongs to E270). Report which pattern you have. And if this code shares the recall history with a ground or polarity code, lead with that when booking — it reorders the whole visit.
The same fault on other brands
On the newer seven-segment Goodman displays the same weak-signal condition reads E6, and the shared Goodman/York legacy boards flash rapid amber for low flame sense current — the flame sensor hub collects the full set. Lennox's edge is instrumentation rather than taxonomy: most boards in that list report that the signal is weak, while this one will show you the microamps.
What this looks like
From the outside, this usually shows up as the burner lights and then drops out a few seconds later — that walkthrough covers what to check in order, and when it stops being a homeowner job.