York TM9V080B12MP11 Error Code Steady On (Any Color): Control Failure

What Does Code Steady On (Any Color) Mean?

The Furnace Control Board on this York two-stage, variable-speed furnace communicates only through flashing patterns — slow green in standby, slow amber during a heat call, or numbered red flashes for faults. A solid, unchanging LED that never blinks means the board's microprocessor has stopped running its program. Because the LED is frozen rather than cycling, the board can no longer manage the ignition sequence, staging, or the safety circuits it is supposed to monitor.

York's guidance is to first rule out a temporary glitch by cycling power: turn the furnace off at the breaker for about thirty seconds, then restore it. If the board reboots into a normal slow-flash pattern, it may have hit a transient upset and recovered. If it goes straight back to steady on, the failure is permanent.

The board is not field-repairable and must be replaced as a unit. This is different from a dark LED, which means the board has no power at all; steady-on means the board is powered but its processor has hung.

What You'll Notice

• The control-board LED glows steadily in one color and never blinks or changes.
• The furnace does not respond normally to the thermostat, or behaves erratically, because the board has stopped executing its program.
• Cycling power at the breaker does not restore a normal slow-flash pattern — the LED returns to steady on.

Common Causes

How This Is Diagnosed

The first step is a power cycle: switch the furnace off at the breaker for roughly thirty seconds and back on, then watch the LED. Recovering to a normal slow-flash pattern suggests a one-time upset, while an immediate return to steady on confirms the board has failed. A technician then verifies that supply voltage and the 24-volt transformer output are correct — ruling out a power-quality problem feeding the board — before condemning and replacing the control board, since the board is not repairable in the field.

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 steady on after a full power-off/power-on reset at the breaker.
• The furnace behaves erratically — starting or stopping without matching the thermostat — while the LED sits solid.
• The steady-on condition keeps recurring even though supply power appears stable.

✓ Verified against manufacturer service manual — March 2026