York TM9V080B12MP11 Error Code Slow Green Flash: Normal Operation - No Thermostat Calls
What Does Code Slow Green Flash Mean?
The TM9V080B12MP11 is a York two-stage, variable-speed ECM condensing gas furnace, and its Furnace Control Board reports status through a single multi-color LED. Green is the board's "healthy" color, and a slow flash is defined as two seconds on and two seconds off, which is deliberately slower than the rapid 1/3-second blinks used for numbered red fault codes. Put together, a slow green flash tells you the electronics are powered, all safety circuits are satisfied, and the furnace is simply idle.
Because this furnace stages between low and high fire, it spends a good deal of time waiting between cycles once the house is up to temperature. During that idle time the board rests at slow green. The moment your thermostat closes a heat call, the same LED changes to a slow amber flash to show it is now responding to a demand. If instead you ever see the LED steady on or flashing red, the board has left standby and is reporting a control failure or a specific fault.
In short, slow green is the pattern you want to see whenever the furnace is not actively heating.
What You'll Notice
- The control-board LED blinks green slowly, roughly two seconds on and two seconds off.
- The furnace is quiet and idle — no inducer, ignitor glow, or blower — while the room is at or above the thermostat setpoint.
How This Is Diagnosed
There is nothing to diagnose, because slow green is a normal, no-fault state. If the house feels cold while the board shows slow green, the check is at the thermostat rather than the furnace: confirm it is set to Heat, that the target temperature is above the current room temperature, and that a schedule or hold is not suppressing the call. If raising the setpoint makes the LED change to slow amber and the furnace begins its ignition sequence, everything is working as designed.
- The board shows a slow green flash but the furnace never starts after you raise the thermostat well above room temperature and confirm it is in Heat mode with fresh batteries.
- The LED keeps alternating between slow green and a numbered red flash code, which points to an intermittent fault rather than simple standby.
Frequently Asked Questions
Is a slow green flash on my York TM9V080B12MP11 a problem?
No. Slow green is the normal standby indication. It means the control board has power and is waiting for the thermostat to call for heat.
Why is my furnace showing a green light but not heating?
A slow green flash specifically means there is no active call for heat, so the issue is usually a thermostat setting rather than the furnace. Confirm the thermostat is in Heat mode and set above room temperature.
What color should the LED be when the furnace is actually running?
When a heat call is active the board switches to a slow amber flash. Green marks a healthy board, and slow amber marks a healthy board that is now responding to a heat demand.
✓ Verified against manufacturer service manual — March 2026