York TM9T100C16MP11 Error Code Steady On (Any Color): Control Failure
What Does Code Steady On (Any Color) Mean?
A steady, non-blinking LED on the York TM9T100C16MP11 control board indicates the control board itself has failed. The LED should always be flashing a pattern (slow green for standby, slow amber for heating, or a numbered red code for faults). When it stays solid, the board's microprocessor has stopped running its program.
Before concluding the board is bad, try a power cycle: turn off the furnace at the circuit breaker, wait 30 seconds, then restore power. Watch the LED carefully as power returns. If it goes through a brief startup sequence and then settles into a normal slow flash, the board may have recovered from a temporary software fault. If the LED goes directly to a steady-on state without blinking, the control board has permanently failed and cannot be repaired in the field.
A qualified HVAC technician must replace the control board with the correct part for this furnace model. After installation, the technician will verify all furnace operations including ignition, flame sensing, blower speeds, and safety switch circuits.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Failed control board | Most common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- When the LED remains steady on after a power cycle — the control board must be professionally replaced
Sources
✓ Verified against manufacturer service manual — March 2026