Lennox SLP98UHV Error Code E 240: Low Flame Current
What Does Code E 240 Mean?
Error code E240 on the Lennox SLP98UHV indicates low flame current detected during run mode. The flame sensor is a metal rod positioned in the burner flame that generates a tiny electrical current (measured in microamps) to confirm the burners are lit. When this current drops below the expected threshold, the control logs this alert.
The furnace may continue to operate with this code, but the flame signal is marginal. If the current drops further, the furnace will lose flame detection entirely, causing it to shut down during operation and eventually enter soft lockout (E270 or E273). This code is an early warning that the flame sensor needs maintenance.
The most common cause is a buildup of oxidation, carbon, or silica residue on the flame sensor rod. Over time, this insulating layer reduces the sensor's ability to conduct the small current needed for flame detection. A secondary cause is a poor unit ground — the flame sensing circuit requires a clean electrical ground path between the furnace chassis and the electrical panel.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty flame sensor rod coated with oxidation buildup | Most common | ✗ Call a pro → |
| Poor ground connection on furnace | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- E240 returns after cleaning the flame sensor
- The flame sensor rod is cracked or physically damaged
- The flame appears weak, yellow, or flickers during operation
- You also see E270 or E273 (soft lockout codes)
- You suspect a grounding issue with the furnace
✓ Verified against manufacturer service manual — March 2026