York TM9T100C16MP11 Error Code Continuous Red Flash: Twinning Error
What Does Code Continuous Red Flash Mean?
A continuously flashing red LED on the York TM9T100C16MP11 indicates a twinning error. Twinning is a configuration where two furnaces are wired together to operate as a single system, sharing a common thermostat signal. This error appears when the 24-volt phasing between the twinned units is incorrect or one of the twinned units has lost power.
If your home has a twinned furnace setup, this code means the communication link between the two units has failed. The wiring that synchronizes the two furnaces may have become disconnected or damaged, or the 24V transformers in the two units may be out of phase with each other. If only one of the twinned units has lost power, the other unit will display this error.
If your furnace is a standalone unit (not twinned), this code should not normally appear and may indicate a control board issue. In either case, a qualified HVAC technician needs to inspect the twinning wiring, verify power to both units, and confirm correct 24V phasing.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Incorrect 24V phasing in twinning wiring | Common | ✗ Call a pro → |
| No power to twinned unit | Common | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- When this code appears — twinning wiring and phasing corrections require professional HVAC service
Sources
✓ Verified against manufacturer service manual — March 2026