Lennox SLP98UHV Error Code E 273: Soft Lockout - Flame Failure
What Does Code E 273 Mean?
The E 273 fault on the Lennox SLP98UHV is a soft lockout that follows repeated flame failures. Unlike E 270, where flame is never confirmed, in E 273 the furnace does light — but the flame signal keeps dropping out during the run. After too many failed recycles the SureLight control locks the furnace out as a safety precaution. As a soft lockout it clears automatically after a wait period, or when a later heat call completes successfully.
The flame sensor is a thin metal rod in the burner flame that conducts a tiny microamp current to prove flame. When the rod becomes coated with carbon or oxidation, that current weakens intermittently until the control can no longer reliably see the flame; it then closes the gas valve believing the flame has gone out, recycles, relights, and loses the signal again. That repeating cycle is exactly what E 273 records.
E 273 is the escalation of E 240 (low flame current). Where E 240 is the early warning that the flame signal is getting weak, E 273 is the lockout that happens once the signal drops out enough times to trip the recycle limit. Because the mechanism is the same, the manual points E 273 to the E 240 troubleshooting path — clean or check the flame sensor and verify the furnace ground.
What You'll Notice
- The furnace ignites, runs briefly, then the flame drops out — repeatedly
- Short cycling followed by a lockout with no heat
- The blower may run on but the burners keep cutting out
- Heat that starts and stops several times before the furnace gives up
- E 273, often alongside E 240, shown in the fault history
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Dirty flame sensor causing repeated flame signal loss | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician confirms the burners are lighting but the flame signal is dropping out, which separates E 273 from the no-flame E 270. They measure the flame sensor's microamp current while the burners fire and, if it is weak or erratic, remove the sensor and clean the rod with a Scotch-Brite pad until the metal is dull-bright, avoiding the porcelain base and not bending the rod, then remeasure.
If the signal stays weak after cleaning, they check the furnace ground and the sensor wiring, since a poor ground produces the same intermittent dropout. A rod that remains marginal despite a clean surface and 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 ignites but the flame goes out within seconds, over and over
- E 273 keeps returning after the furnace has been reset
- You also see the E 240 low-flame-current code
- The flame looks yellow or orange instead of blue during the brief run
- The furnace displays other error codes alongside E 273
Frequently Asked Questions
Can I clean the flame sensor myself to fix E 273?
On this sealed communicating furnace it's best handled by a technician, who will also measure the microamp signal and check the ground rather than only wiping the rod. Since E 273 can also stem from a grounding problem, a professional check confirms whether cleaning alone will actually resolve it.
How is E 273 different from E 270?
In E 273 the burners light but the flame keeps dropping out during the run, so it's a flame-failure lockout tied to weak flame sensing. In E 270 flame is never confirmed at all after the ignition retries. The difference tells a technician whether to focus on flame detection or on the ignite-and-prove chain.
Why does this keep coming back after it clears on its own?
E 273 is a soft lockout, so it releases after a wait or a successful cycle — but if the underlying dirty sensor or weak ground isn't corrected, the flame signal drops out again and the code returns. Addressing the root cause is what stops the repeat lockouts.
✓ Verified against manufacturer service manual — March 2026