York TM9T100C16MP11 Error Code Steady On (Any Color): Control Failure
What Does Code Steady On (Any Color) Mean?
The control board on the York TM9T100C16MP11 normally communicates by flashing its LED — green and amber for status, red counts for faults. When the LED comes on solid and never flashes, the board is reporting that its own internal control logic has failed. Unlike a dark LED, which means no power reaches the board, a steady-on LED means power is present but the board can no longer operate correctly.
Because this integrated board manages the two-stage gas valve, ignition sequence, and blower on the TM9T, a control failure disables heating and can affect cooling as well, since the board also drives the indoor blower for the air conditioner.
York instructs turning the furnace power off and back on once. A brief electrical glitch can occasionally latch the board, and a single power cycle may clear it. If the LED returns to a solid state after cycling power, the fault is permanent — York specifies that the board is not field-repairable and must be replaced.
What You'll Notice
- The board LED glows solid (steady) in any color instead of flashing.
- The furnace does not respond to the thermostat and produces no heat.
- Cycling the furnace power off and on does not restore a normal flashing pattern.
- Cooling may also be affected because the board controls the indoor blower.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Failed control board | Most common | ✗ Call a pro → |
How This Is Diagnosed
The one homeowner-safe step is to turn the furnace power off at the breaker, wait about 30 seconds, and turn it back on to see whether the board resumes a normal flashing pattern. If the LED comes back solid, the diagnosis is a failed board. A technician will first confirm the incoming line voltage and transformer output are correct — ruling out power-quality problems — before replacing the board, which York states is not field-repairable.
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 LED returns to a solid, non-flashing state after you cycle the power
- The furnace will not respond to the thermostat after a power cycle
- Cooling is also affected, pointing to the shared control board
- The furnace repeatedly loses all normal operation with a solid LED
Frequently Asked Questions
Can I fix a control failure on my York TM9T myself?
No. York states the control board is not field-repairable. The only homeowner step is to cycle the power once; if the LED stays solid, a technician must replace the board.
Will turning the power off and on fix a steady-on LED?
It may clear a one-time electrical glitch. If the solid LED returns after a power cycle, the board has genuinely failed and needs replacement.
What is the difference between a steady-on LED and no LED at all?
A steady-on LED means the board is powered but has failed internally, while no LED means the board is receiving no power. The two point to very different causes.
Sources
✓ Verified against manufacturer service manual — March 2026