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.