York TM9T100C16MP11 Error Code LED Steady Off: No Power / Blown Fuse
What Does Code LED Steady Off Mean?
When the LED on the York TM9T100C16MP11 control board is completely off, the board is not receiving electrical power. The furnace cannot operate in any mode — no heating, cooling, or fan — until power is restored to the control board.
The most common cause is a loss of power to the furnace: a tripped circuit breaker, a blown fuse in the electrical panel, or the furnace power switch being turned off. Check your electrical panel first and reset the breaker if it has tripped. Also verify that the furnace power switch (typically a light switch mounted on or near the furnace) is in the on position.
If the breaker and switch are both fine, the problem is internal to the furnace. A blown fuse on the control board itself is common — these small fuses protect the board from short circuits and can blow due to a wiring fault or failed component elsewhere in the system. A failed control board is also possible. Both of these conditions require a qualified HVAC technician to diagnose and repair.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| No power to control board | Most common | ✗ Call a pro → |
| Blown fuse on control board | Common | ✗ Call a pro → |
| Failed control board | Uncommon | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- If the breaker is not tripped and the furnace switch is on but the LED remains off — the board fuse, transformer, or board itself may have failed
Sources
✓ Verified against manufacturer service manual — March 2026