York Furnace 9 Red Flashes (YP9C Modulating)
Hard lockoutShown as: Nine red flashes, two-second pause, repeating
Reversed line polarity or improper grounding at the furnace.
Manual wording: “Nine Red Flashes - Reversed line polarity or improper grounding. Check polarity of the incoming power to the furnace. Check the grounding of the furnace, including the transformer ground and the L1 and neutral connections.”
Documented for: YP9C, YPLC modulating furnaces
Likely causes, most common first
- Line voltage hot and neutral reversed
- Poor or missing furnace ground
- Bad transformer ground or L1/neutral connection
What you can safely check yourself
- If this appeared right after electrical work, mention that to the technician
- Check the furnace breaker is fully on
When to call a technician
Polarity and grounding are line-voltage electrical work. Have a technician or electrician correct it.
A York furnace throwing this code repeatedly has a reason — clearing the code without fixing the cause just hides it.
Source
- Installation Manual - Modulating ECM Residential Gas Furnace with ECM Motor, Models YP9C*C / TP9C*C / CP9C*C / LP9C*C (Up to 98% Modulating Multi-position), 1083292-UIM-A-0114 (Johnson Controls Unitary Products (York), YP9C*C (York Affinity modulating), and the identical TP9C / CP9C / LP9C chassis) · checked 2026-08-25