Lennox EL296UHV Error Code E 240: Low Flame Current
What Does Code E 240 Mean?
Error code E240 on the Lennox EL296UHV means the control detected low flame current during run mode. The flame sensor is a metal rod that sits in the burner flame and passes a tiny current, measured in microamps, that tells the SureLight control the burners are actually lit. When that current falls below the expected level while the furnace is running, the board logs E240 as an early warning.
The furnace may keep running with E240 present, but the flame signal is marginal. If the current keeps dropping, the control can lose flame detection entirely and shut down mid-cycle; repeated flame-loss events then drive the board into E273, a soft lockout after too many recycles ended by flame failure. Treating E240 promptly usually prevents that escalation.
The most common cause is a buildup of oxidation and combustion residue on the flame-sensor rod, which insulates it and weakens the current it can pass. A poor unit ground is the other common contributor, because the flame-sensing circuit needs a clean ground path to read correctly. E240 sits just ahead of E270 (soft lockout after ignition failures with no flame detected) and E273 in the flame-detection family on this board; the alert clears after the current heat call completes.
What You'll Notice
- The furnace runs but occasionally drops out mid-cycle before restarting
- The seven-segment display shows E240
- Heating cycles feel shorter or less consistent than usual
- The problem tends to worsen gradually over a season as residue builds up
- E273 (flame-failure soft lockout) may start appearing alongside E240
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty flame sensor | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician measures the flame-sensor current in microamps through the control's diagnostics or field-installed test mode while the burners are lit, comparing the reading against the expected range for the EL296UHV. A low reading with an otherwise strong, stable flame points to a fouled sensor rod; the technician cleans the rod or replaces the sensor as part of the repair.
They also verify the unit ground, typically by checking the voltage between neutral and ground, because a weak ground can lower the sensed current even when the sensor is clean. If cleaning and a good ground do not restore normal microamps, the sensor is replaced. This is diagnostic information only; measuring flame current and servicing the burner-area sensor are technician tasks, not homeowner repairs.
When to Call a Professional
This code involves components that are not homeowner-serviceable, so have a licensed HVAC technician diagnose and repair it. Keep in mind:
- The display shows E240 and the furnace drops out during heating
- E273 (flame-failure soft lockout) starts appearing along with E240
- The flame-sensor rod looks cracked, warped, or heavily coated
- The furnace heats inconsistently and cycles more often than it used to
- You suspect a grounding problem, such as other electrical anomalies in the home
Frequently Asked Questions
Can the furnace keep running with E240 showing?
It often can for a while, because E240 is an early warning that the flame signal is weak rather than an immediate shutdown. But the reading tends to keep dropping until the furnace loses flame detection and eventually locks out on E273, so it should be serviced.
Why would a clean-looking flame sensor still read low?
A thin oxidation film can insulate the rod without looking obviously dirty, and a poor unit ground can lower the sensed current even when the rod is clean. A technician measures the actual microamps and checks the ground to tell which it is.
Is E240 an emergency?
It is rated lower severity and is not an immediate hazard, but ignoring it usually leads to nuisance shutdowns and an E273 lockout. Having it diagnosed early keeps the furnace heating reliably.
✓ Verified against manufacturer service manual — March 2026