Thermostat displays E9 and may disable certain functions; often associated with accessory modules like humidifiers or ventilators.

Safety first: Turn off power to the HVAC system at the breaker before working on accessory wiring.

• Step 1 – Verify accessory settings: In the thermostat's advanced menu, confirm that only installed accessories (humidifier, dehumidifier, ventilator) are enabled.
• Step 2 – Inspect wiring at thermostat: Remove the thermostat and check HUM, DHUM, and VENT terminals. Ensure wires are secure and not shorted to adjacent terminals.
• Step 3 – Inspect wiring at equipment: At the furnace/air handler, verify that accessory wires are correctly landed on the Carrier control board or accessory terminals per the installation manual.
• Step 4 – Isolate the fault: Temporarily disconnect accessory wires at the thermostat, then power up the system. If E9 clears, reconnect accessories one at a time to identify the faulty device or circuit.
• Step 5 – Replace failed accessory: If a specific humidifier, dehumidifier, or ventilator module causes E9 when connected, that accessory likely needs replacement.
• When to call a pro: Accessory modules often require configuration and airflow verification; have a Carrier technician complete replacement and setup.