York TM9T100C16MP11 Error Code Slow Green Flash: Normal Operation - No Thermostat Calls
What Does Code Slow Green Flash Mean?
On the York TM9T100C16MP11, the integrated furnace control board uses a single LED to report status. A slow green flash — roughly two seconds on and two seconds off — is the board's way of saying it has finished its self-diagnostics, found no fault conditions, and is simply waiting in standby. York manufactures this two-stage furnace to sit in this state whenever the thermostat is satisfied.
This indication is easy to tell apart from the slow amber flash, which is the normal state while the furnace is actively servicing a call for heat. If your LED is green and slow, the thermostat is not currently requesting heat. That is completely normal between heating cycles or when the room has reached the setpoint.
If you expected heat but see this code, the issue is upstream of the furnace — usually the thermostat. Confirm it is set to Heat, that the target temperature is above the current room reading, and that any programmable schedule or dead thermostat batteries have not lowered the setpoint.
What You'll Notice
- The control board LED blinks green with a slow, steady rhythm of about two seconds on and two seconds off.
- The furnace is quiet and producing no heat because the thermostat is satisfied or set below the current room temperature.
How This Is Diagnosed
There is nothing to repair for this code — it is the healthy standby indication. To confirm it is behaving correctly, set the thermostat to Heat with a target a few degrees above the room temperature and watch the board: within a short time the LED should change to a slow amber flash and the furnace should begin a heating cycle. If it does, the furnace and board are fine and the earlier green flash simply meant no heat was being requested.
- The LED stays on a slow green flash even though the thermostat is set to Heat and is calling for a temperature above the current room reading
- You want a seasonal safety and efficiency tune-up before the heating season, even though no fault is present
Frequently Asked Questions
Is a slow green flash on my York TM9T a problem?
No. It is the normal standby indication, meaning the board is powered, healthy, and simply waiting because the thermostat is not calling for heat.
Why is my furnace not heating if the light is flashing green?
A slow green flash specifically means there is no active call for heat. Check that the thermostat is in Heat mode and set above the room temperature; when it calls for heat the LED changes to a slow amber flash.
What is the difference between the slow green and slow amber flash?
Slow green means standby with no call for heat, while slow amber means the furnace is running normally in response to a heating request. Both are normal indications.
Sources
✓ Verified against manufacturer service manual — March 2026