Lennox SLP98UHV Error Code E 240: Low Flame Current
What Does Code E 240 Mean?
The E 240 fault on the Lennox SLP98UHV means the SureLight control measured low flame current while the burners were running. The flame sensor is a thin metal rod placed in the burner flame that passes a tiny current — measured in microamps — to prove the flame is actually present. When that current drops below the expected threshold, the board logs E 240 as a warning that flame detection is becoming marginal.
The furnace may keep running with E 240 present, but the signal is weak and getting weaker. This is an early-warning code: if the current keeps falling, the board will eventually lose flame detection during a run and shut down. That is how E 240 escalates into E 273 (soft lockout because the flame keeps failing) and, if the flame is never confirmed at all after repeated retries, into E 270 (soft lockout with no flame sensed). Catching it at the E 240 stage usually means a simple cleaning rather than a no-heat lockout later.
The most common cause is a film of oxidation, carbon, or silica residue building up on the sensor rod, which insulates it and chokes off the microamp signal. A secondary cause is a poor unit ground — the flame-sensing circuit needs a clean electrical ground path between the furnace chassis and the electrical panel, and a weak ground can starve the reading even when the rod is clean.
What You'll Notice
- The furnace lights normally, then shuts down a short time later
- Short cycling — the burners run briefly and cut out, then try again
- Heat that starts fine but becomes intermittent or inconsistent
- No fault at rest, but E 240 shows in the history after a run
- Occasional E 270 or E 273 lockouts appearing alongside E 240
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 → |
How This Is Diagnosed
A technician measures the flame sensor's microamp signal with a meter while the burners are firing and compares it to the expected value. If the current is low, they typically remove the flame sensor and clean the rod with a Scotch-Brite pad until the metal is dull-bright, taking care not to touch the porcelain base or bend the rod, then remeasure the signal.
If cleaning does not restore the reading, the technician checks the furnace grounding — measuring neutral-to-ground and confirming a solid chassis ground — since a poor ground produces the same low-current symptom. A sensor that stays weak after cleaning and with a good ground is replaced.
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 furnace lights and then drops out or short-cycles
- E 240 keeps returning even though the flame looks normal
- You also see E 270 or E 273 lockout codes
- The flame appears weak, yellow, or flickers during operation
- You suspect a grounding problem with the furnace
Frequently Asked Questions
Can I clean the flame sensor on my Lennox SLP98UHV myself?
On this sealed communicating furnace the flame sensor work is best handled by a technician, who will also verify the microamp reading and the unit ground rather than just wiping the rod. Because E 240 can be a symptom of a grounding issue as well as a dirty sensor, a professional check confirms the real cause.
Is E 240 an emergency?
It is not an immediate safety hazard the way a rollout or gas-valve code is, but it is an early warning. Left alone, the weak flame signal tends to worsen and push the furnace into an E 270 or E 273 no-heat lockout, so it is worth addressing before the next cold spell.
Why does a dirty rod cause a low reading?
The sensor works by conducting a very small current through the flame. Oxidation, carbon, or silica buildup on the rod acts as an insulator and chokes that current, so the board reads it as low even though the burners are lit.
✓ Verified against manufacturer service manual — March 2026