York TM9T100C16MP11 Error Code 9 Red Flashes: Reversed Line Voltage Polarity / Grounding Problem
What Does Code 9 Red Flashes Mean?
A 9 red flash code on the York TM9T100C16MP11 indicates the control board has detected reversed line voltage polarity, a grounding problem, or reversed low voltage transformer wires. Both heating and cooling operations are completely disabled until this electrical issue is resolved. The furnace will not start its ignition sequence.
Reversed polarity means the hot and neutral wires at the furnace's electrical connection are swapped. This can happen during installation, after electrical panel work, or if someone serviced the furnace and reconnected the wires incorrectly. A grounding problem — where the furnace chassis is not properly connected to the building's electrical ground — is equally disabling. The flame sensor can also cause this code if it is shorted to the furnace chassis, creating a false ground path.
A qualified electrician or HVAC technician needs to check the polarity at the furnace's electrical connection and at the branch circuit, verify the furnace chassis ground, and inspect the flame probe for contact with the chassis. All wiring corrections must be made before the furnace will operate.
Common Causes
| Cause | Likelihood | DIY? |
|---|---|---|
| Reversed line voltage polarity | Most common | ✗ Call a pro → |
| Furnace grounding problem | Common | ✗ Call a pro → |
| Flame probe shorted to chassis | Uncommon | ✗ Call a pro → |
When to Call a Professional
Contact a licensed HVAC technician if:
- When this code appears — electrical polarity and grounding corrections require a licensed technician
Sources
✓ Verified against manufacturer service manual — March 2026