York TM9T100C16MP11 Error Code Continuous Red Flash: Twinning Error
What Does Code Continuous Red Flash Mean?
Twinning is the practice of linking two furnaces so they operate together from a single thermostat, typically to serve a large open area. For twinned York TM9T units to run in step, both control boards must share correctly phased 24-volt power. A continuous red flash — the LED flashing on and off with no pauses — tells you the board has detected incorrect 24-volt phasing in the twinning wiring or that one of the twinned units has no power.
This code is meaningful only in a two-furnace, twinned installation. If your home has a single furnace and this code appears, it usually means the twin terminals on the board have been miswired, and a technician should inspect the low-voltage connections.
Do not confuse this with the 9 red flashes code, which reports reversed line-voltage polarity or a grounding problem on a single 120-volt furnace. The continuous red flash is specifically about the 24-volt link between two twinned units.
What You'll Notice
- A continuous red flash with no pauses appears on one or both control boards.
- The condition is present in a dual/twinned furnace installation sharing one thermostat.
- One of the twinned furnaces may run while the other does not.
- Heating output is uneven or inconsistent between the two linked units.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Incorrect 24V phasing in twinning wiring | Common | ✗ Call a pro → |
| No power to twinned unit | Common | ✗ Call a pro → |
How This Is Diagnosed
A technician first confirms that the system is genuinely a twinned pair, then verifies both furnaces have power at their disconnects and breakers. Next the technician checks the 24-volt phasing across the twin terminals so the transformers are in phase, and inspects the twinning harness for correct polarity and solid, continuous connections. On a single-furnace system showing this code, the technician looks for accidental miswiring at the twin terminals.
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:
- You have two furnaces sharing one thermostat and one or both show a continuous red flash
- One of the twinned furnaces has no power or is tripping its breaker
- The code appears on a single-furnace system, suggesting a wiring error at the twin terminals
Frequently Asked Questions
What is a twinning error on a York TM9T?
Twinning links two furnaces to run together from one thermostat. A twinning error means the 24-volt wiring between them is mis-phased or miswired, or one unit has lost power.
I only have one furnace — why am I seeing a continuous red flash?
On a single-furnace system this usually points to miswiring at the board's twin terminals. A technician should inspect the low-voltage connections.
Can I fix twinning wiring myself?
No. Correcting 24-volt phasing between two furnace boards requires a technician to avoid creating a transformer short or unsafe operation.
Sources
✓ Verified against manufacturer service manual — March 2026