York TM9T100C16MP11 Error Code 1 Red Flash: Flame Sensed Without Call for Heat
What Does Code 1 Red Flash Mean?
The York TM9T100C16MP11 constantly watches its flame sensor. A 1 red flash code means it detected flame at a time when the thermostat was not calling for heat and the gas valve should have been fully closed. This is a safety condition: to clear any combustion, the board turns on both the inducer motor and the supply air blower to ventilate the furnace.
The most likely cause is a gas valve that is leaking internally or closing too slowly at the end of a cycle. The valve's internal seat and solenoid can wear over time, allowing a small amount of gas to seep past and ignite on the still-hot burner surfaces.
It is important to distinguish this from the 10 red flashes code. A 1 red flash means flame was actually sensed with no call for heat, while 10 red flashes means the gas valve is being electrically energized with no call for heat even though no ignition sequence has started. Both are gas-related safety codes, but they point to different failures — the flame itself versus the valve's electrical command.
What You'll Notice
- The blower and inducer run even though the thermostat is not calling for heat.
- The burner appears lit, or you smell combustion, when the system should be idle.
- The board shows a single red flash repeated with a pause.
- The furnace will not settle back into the slow green standby flash.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Leaking or slow-closing gas valve | Most common | ✗ Call a pro → |
How This Is Diagnosed
A technician isolates the gas supply and then inspects the gas valve for internal seepage or a slow-closing seat, which is the manual's primary suspect. The flame sensor and its wiring are also checked to rule out a false flame signal or a probe shorted to the chassis. Because every likely cause on this code involves the gas valve or the burner, the work is diagnostic and repair by a professional only.
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 blower and inducer run with no thermostat call for heat
- You smell gas or combustion odors around the furnace
- The burner stays lit after the heating cycle should have ended
- The single red flash returns after the furnace power is cycled
Frequently Asked Questions
Is a 1 red flash on my York TM9T dangerous?
It can be, because it often points to a gas valve leaking or closing slowly. Shut the furnace off and call a technician promptly; if you smell gas, leave immediately and call your gas company from outside.
What is the difference between 1 red flash and 10 red flashes?
A 1 red flash means flame was actually detected without a call for heat, while 10 red flashes means the gas valve is being energized without a call for heat even though no ignition sequence has begun.
Can I keep using the furnace until it is repaired?
No. Because a leaking gas valve is a safety hazard, turn the furnace off at the breaker and have it inspected before running it again.
Sources
✓ Verified against manufacturer service manual — March 2026