American Standard Thermostat Manuals

Most Common Error Codes & Alerts (Communicating & Smart Models)

• Hard Lockout / System Lockout: Critical fault detected – check furnace/air handler for codes; possible flame sensor, pressure switch, or limit switch issue; power cycle thermostat & equipment.
• Comm Failure / No Communication: Lost link between thermostat & indoor/outdoor unit – check 4-wire communicating bus wiring (R, B, D+, D-); verify polarity; reset system by removing power for 1 min.
• Outdoor Temp Sensor Failure: ODT sensor open/short – check wiring at outdoor unit; replace sensor if faulty.
• Indoor Temp Sensor Failure: Thermostat internal sensor fault – use remote sensor if equipped; may require thermostat replacement.
• Humidity Sensor Failure: Humidifier/dehumidifier control issue – check humidity sensor wiring; verify humidistat settings.
• Low Battery / Replace Battery: AA batteries low (non-powered models) – replace batteries; check common wire (C) connection on powered models.

Wi-Fi / Connected / Nexia Specific (2020–2026 Models)

• Wi-Fi Offline / Not Connected: Thermostat not on network – verify 2.4 GHz Wi-Fi (not 5 GHz); re-enter credentials via app or thermostat screen; restart router.
• App Control Lost: Check internet connection; re-link thermostat in Nexia/ComfortLink app; ensure firmware is updated.
• Schedule Not Following: Verify Away/Geo-fencing settings; check time zone & DST; re-sync with app.

General Thermostat Specific

• Not Cooling / Heating: Check mode (Cool/Heat/Auto); verify set point; inspect breaker/fuse at HVAC unit; check filter & airflow.
• Short Cycling: Dirty filter, low refrigerant, or oversized system – clean/replace filter; check for error codes on equipment.
• Blank Screen / No Power: Check 24V power (R & C wires); replace batteries if applicable; inspect transformer & fuse on control board.
• Fan Runs Continuously: Fan switch set to ON instead of AUTO; possible stuck relay; check wiring.

Safety note: Always turn off power at breaker before servicing thermostat wiring or HVAC equipment. High-voltage (120/240V) and low-voltage (24V) circuits present. Never bypass safety controls. For gas/electric system faults, contact a licensed HVAC technician.