Fault Code 10: Faulty HX OT1 switch – open when cool. Lead disconnected from HX OT1. Maximum room fan speed set too low during set‐up. Thermistor in zoned‐off duct branch. Thermistor (2‐stage & modulating models) too far from heater outlet. Low room air flow due to user selection of low fan speed or automatic selection of low fan speed in zoned operation (2‐stage and modulating models). Low room air flow.