York TM9T100C16MP11 Error Code 9 Red Flashes: Reversed Line Voltage Polarity / Grounding Problem
What Does Code 9 Red Flashes Mean?
A 9 red flashes code on the York TM9T100C16MP11 means the board has detected reversed line-voltage polarity, a grounding problem, or reversed low-voltage transformer wires. Both heating and cooling are completely disabled, and the furnace will not start its ignition sequence until the issue is corrected.
The reason this matters is that flame rectification depends on a correct electrical reference: the board senses flame by comparing the flame signal against neutral and a solid chassis ground. If hot and neutral are swapped, or the furnace is not properly grounded, that reference is lost and the board cannot safely prove flame. A flame probe that is shorted to the chassis can also create a false ground path and trigger this code.
Reversed polarity typically appears after electrical panel work, a receptacle change, a new installation, or a service call where wires were reconnected incorrectly. This is a line-voltage issue and is distinct from the continuous red flash, which is a 24-volt twinning problem between two furnaces.
What You'll Notice
- Neither heating nor cooling works — both are disabled.
- The board shows nine red flashes.
- The problem often appears right after electrical work or a new installation.
- An outlet tester may show reversed polarity on nearby receptacles.
- The furnace never begins its ignition sequence.
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 → |
How This Is Diagnosed
A technician or electrician checks the line polarity at the furnace connection and at the branch circuit to confirm hot and neutral are correct, verifies the furnace chassis is properly grounded, and inspects the transformer's primary and secondary wiring. The flame probe is also checked to make sure it is not shorted to the chassis. All wiring corrections must be completed before the furnace will operate.
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:
- Neither heating nor cooling works and the board shows nine red flashes
- The code appeared right after electrical work or a new furnace installation
- An outlet tester shows reversed polarity on nearby receptacles
- You are not qualified to verify line-voltage polarity and grounding
Frequently Asked Questions
What does reversed polarity mean on my York TM9T?
It means the hot and neutral wires at the furnace's line connection are swapped. The furnace needs a correct electrical reference to sense flame safely, so it disables itself until this is fixed.
Can I fix a 9 red flashes code myself?
No. Correcting line-voltage polarity and grounding is electrical work that should be done by a licensed electrician or HVAC technician.
How is 9 red flashes different from the continuous red flash?
Nine red flashes is a 120-volt polarity or grounding problem on a single furnace, while the continuous red flash is a 24-volt twinning fault between two furnaces wired together.
Sources
✓ Verified against manufacturer service manual — March 2026