York furnace 1 red flash: Flame sensed with the gas valve off
One red flash means flame is being detected while the gas valve is off. Either gas is burning when it should not be, or the sensing circuit is reporting a flame that is not there.
Applies to: York, Luxaire and Coleman gas furnaces using the UTEC integrated furnace control boards documented in technical service letter ST-045-09. This code is stored in the error code array.
What it means
The control watches the flame sensor continuously, and detecting flame with the gas valve commanded off produces this code. It is treated seriously because one of the two possible causes is a gas valve that is not closing fully, meaning gas entering the combustion chamber outside a controlled cycle. The other is electrical: a short in the flame sensing circuit produces a signal the control cannot distinguish from a real flame. Since it cannot tell them apart, it assumes the dangerous one. York marks this code as stored in the error array, so it can be recovered from the history even after the furnace appears to recover.
Most common causes
- A gas valve leaking through or not closing fully.
- A slow-closing gas valve, where flame lingers after shutdown.
- A short to ground in the flame sensor wiring or at the sensor.
- A chafed or damaged sensor wire touching the cabinet.
- A failed control board misreading the flame signal.
- Residual flame on a burner after the main flame is cut.
How to diagnose it
- Confirm the colour is red. Amber patterns on these boards indicate normal heating states.
- Do not open the burner compartment while the furnace is powered.
- Note any smell of gas and act on it immediately rather than continuing.
- Have the stored fault history replayed, since this code is retained and its position in the sequence is informative.
- Last-generation White-Rodgers Emerson boards word this one differently — flame sensed when no flame should be present — but it is the same fault and the same repair.
How to fix it
- If you smell gas: leave the building, operate no electrical switches, and call your gas utility's emergency line or the fire service from outside.
- With no gas smell, switch the furnace off at its service switch and leave it off until a licensed technician has seen it.
- This is entirely technician work, involving the gas valve and the flame-sensing circuit.
- Do not reset repeatedly. If the valve is passing gas, each cycle is another opportunity for it to accumulate.
Notes
The wiring-short version is far more common than a leaking gas valve, and a technician usually finds it quickly. That does not make it safe to assume — the control cannot distinguish them and neither can anyone without test equipment at the valve.
Related
- 10 red flashes — Gas valve energised with no call for heat
- 8 red flashes — Lockout on recycles, flame dropouts
- Which York control board you have — York used two control board generations with different code meanings
All codes for this manufacturer
York, Luxaire and Coleman furnace flash codes — every code, with what differs between models.
Not sure this is your brand?
The same count means something else on other furnaces. What 1 flash means on every brand puts them side by side.
What the fix costs
Two very different bills hide behind this code, which is why it needs a proper diagnosis rather than a parts guess. If the gas valve really is leaking through or closing slowly, gas valve replacement runs $300–900 installed on the furnace repair cost guide's figures — and the guide is blunt that a gas valve is never a DIY part. If the fault is the far more common wiring-short version — a chafed flame sensor lead grounding against the cabinet — the fix is often little more than the diagnostic visit itself, typically $75–150 and usually credited against the work. A control board misreading the flame signal is the expensive outlier at $400–900 installed. Nobody can tell you which of the three you have without instruments at the valve and the sensing circuit, which is exactly why the control locks out instead of guessing.
Fix it yourself or call a professional?
This is one of the York codes with no homeowner lane at all. The useful homeowner actions are the ones already listed: act on any gas smell from outside the building, switch the furnace off at its service switch, and leave the burner compartment closed. Everything this code can actually be — a valve passing gas, a shorted flame-sense circuit, a board fault — is either combustion work or live-cabinet diagnosis. Do not air out the room and try again to see if it recurs: the code is stored in the error array, so the board already keeps that record, and if the valve is the cause each retry gives gas another chance to accumulate. A furnace that has set this code has earned a technician visit even if it seems to recover.
The same fault on other brands
Flame detected when no flame should be present has its own code in most control families: Carrier's status 22, Lennox's alert 241, and the Goodman GMEC96's 1 red flash — that last one a near-twin of this code, from the White-Rodgers board lineage the two tables share. The site's flame sensor code index collects twenty-two such codes across eleven control families. York itself splits the gas valve's misbehaviour across two codes: this one for flame sensed with the valve off, and 10 red flashes for the valve energised with no call for heat — the same component approached from opposite directions.
What this looks like
From the outside, this usually shows up as the furnace turns over but never lights — that walkthrough covers what to check in order, and when it stops being a homeowner job.