Honeywell thermostat: Alert code 177 or 178: wired sensor reading

What Honeywell Home says to do

Wiring and power. Honeywell Home ties this, on at least some systems, to the wiring, the power supply or the C wire. The steps here are only Honeywell Home’s own, as its page states them; semiatic gives no wiring instructions of its own. Honeywell Home’s own line on calling a professional is step 2 below.

1. Honeywell Home says to check the indoor sensor (177) or outdoor sensor (178) and its sensor wires.
2. Honeywell Home says to verify the installer settings match the sensor used: 10K or 20K, ISU 500 and 516 for a wired indoor sensor, ISU 130 for a wired outdoor sensor.

A similar symbol on other thermostats

The same symbol on another maker’s thermostat can mean something else.

• tado: Radiator thermostat E1: valve closed position not found — Calibration failed: the closed position of the valve was not detected.
• tado: Radiator thermostat E2: no contact with the valve pin — Calibration failed: no contact was detected between the motor and the valve pin.
• tado: Radiator thermostat E4: valve travel too short — Calibration failed: the distance between the valve’s open and closed positions is too short, which can mean the valve is blocked by calcification (limescale).

Other Honeywell Home thermostat states

• “Cool On” or the snowflake blinking
• “Heat On” or the flame blinking
• “Wait” or “Waiting for Equipment”
• Blank display
• Alert code 164: heat pump problem
• Alert code 170: memory failure
• Alert code 171: set the date and time
• Alert code 173: internal sensor failure
• Alert code 181 or 182: replace filter
• BATT with code 405: low battery
• Only BATT on the screen, code 407: critically low battery
• All Honeywell Home thermostat states on one page