Coleman-Mach HI fault code — Thermostat
Applies to: 9420-381, 9420-382
What HI means
The Coleman-Mach digital thermostat manual says HI displays when current temperature is above 99 F (37 C).
Owner-safe checks
- Compare the thermostat display with room conditions.
- Record whether HI appears continuously or only during heat exposure.
- Avoid changing wiring or internal thermostat settings.
Service-only work
- Sensor faults, thermostat wiring, PTC fuse issues, and HVAC control diagnosis belong to trained technicians.
Safety boundary
Disconnect power before thermostat or HVAC service. Do not probe control wiring unless qualified.