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Error code reference

Lennox Furnace Error Code 1 Flash

Flame present with gas valve de-energized (control 103085).

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What the 1 Flash error code actually means

Your Lennox furnace is detecting a flame inside the burner even though it has tried to shut the gas off, which it recognizes as a potentially dangerous situation. This usually means either a faulty flame sensor is sending a false signal, or the gas valve is not fully closing and allowing gas to keep burning. Either way, the furnace has locked itself out for safety, and the issue should be inspected by a technician before the system is restarted.

Diagnostic procedure

The full 1 Flash procedure

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1
First diagnostic step
Check flame sensor for contamination or short; verify gas valve is not leaking.
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Most common causes of 1 Flash

Ranked by how often they cause this code
  1. 1Contaminated or shorted flame sensor
  2. 2Leaking gas valve
  3. 3Flame sensor wiring short
  4. 4Faulty control board 103085

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Questions people ask about 1 Flash

What does 1 Flash mean on a Lennox furnace?
The 1 Flash code on a Lennox furnace with control board 103085 means the board is detecting a flame signal while the gas valve is de-energized. This indicates either a false signal from a contaminated or shorted flame sensor, or that the gas valve is leaking and allowing continued combustion after being commanded to close. The furnace shuts down as a safety response to this condition.
Can I fix the 1 Flash error on my Lennox furnace myself?
A knowledgeable DIYer can inspect and clean the flame sensor rod, which is the most common cause and requires only basic hand tools. However, diagnosing or replacing a leaking gas valve involves working with gas supply lines and should be performed by a licensed technician. Any gas valve leak test requires proper instrumentation such as a manometer.
How much does it cost to repair a Lennox furnace showing the 1 Flash error?
Repair costs vary depending on the root cause. Cleaning the flame sensor is typically a low-cost service call, often in the range of $75 to $150 for labor. Replacing a faulty flame sensor generally runs $100 to $250 including parts and labor. If the gas valve requires replacement, costs can range from $300 to $600 or more depending on the valve model and local labor rates.
Is it safe to run my Lennox furnace when it shows the 1 Flash error?
You should not continue operating the furnace until this error has been diagnosed. If the cause is a leaking gas valve, the system could sustain combustion when it should be off, which is a genuine safety hazard. Shut the furnace down and have a qualified technician inspect the gas valve and flame sensor before restarting the unit.
Will resetting my Lennox furnace clear the 1 Flash error?
Resetting the furnace may temporarily clear the fault and allow the system to attempt a new ignition cycle, but the error will return if the underlying cause has not been corrected. If the gas valve is leaking or the flame sensor is shorted, the control board will detect the same abnormal condition and shut down again. A reset alone is not a fix.

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