York TM9V080B12MP11 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 TM9V080B12MP11 indicates the control board has detected one of three electrical wiring problems: reversed line voltage polarity, a furnace grounding problem, or reversed low voltage transformer wires. The furnace will not start its ignition sequence until the wiring issue is corrected, and both heating and cooling operations are affected.
Reversed polarity occurs when the hot and neutral wires are connected to the wrong terminals at the furnace's electrical junction box. This is most commonly an installation error or can happen after electrical work in the home. A grounding problem means the furnace chassis does not have a solid connection to the building's electrical ground system. The flame probe being shorted to the furnace chassis can also trigger this code, as it creates an abnormal ground path that the control board interprets as a wiring fault.
A licensed electrician or HVAC technician must check the line voltage polarity at the furnace connection point and at the circuit breaker, verify the ground wire has a low-resistance path to the panel ground, and inspect the flame probe for physical contact with the chassis.
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 professional service
✓ Verified against manufacturer service manual — March 2026