Honeywell T9 Smart Thermostat with Smart Room Sensor NO-COOL Error Fix: Thermostat Calling for Cool but AC Not Running

Error code NO-COOL on the Honeywell T9 Smart Thermostat with Smart Room Sensor means: Thermostat Calling for Cool but AC Not Running. The thermostat shows it is calling for cooling (snowflake icon solid, "Cool On" solid) but the HVAC system is not producing cold air — check that the outdoor unit breaker is on, listen for the outdoor contactor clicking, verify the Y wire is connected properly at both the thermostat and the air handler, check for a blown fuse on the control board, and confirm the condensate drain safety switch has not tripped

The most successful fix is "Test and Replace Control Board Fuse and Verify Condensate Drain Safety Switch" (medium difficulty, ~30 minutes). Estimated DIY cost: $10-$20. HomeMD has 2 community-ranked fixes for this error.

This error is rated medium severity. It should be addressed soon, but your appliance may still function in a limited capacity.

Yes — the top-ranked community fix ("Test and Replace Control Board Fuse and Verify Condensate Drain Safety Switch") is rated medium difficulty and takes about 30 minutes. Tools needed: Multimeter, Phillips screwdriver, Voltage tester.

DIY cost: $10-$20. Professional repair typically costs $130-$200. Many homeowners save money by trying the top-ranked community fix first.

The Honeywell T9 Smart Thermostat with Smart Room Sensor has 8 other known error codes, including: APP-OFFLINE (Thermostat Shows Offline in App), AUX-HEAT ("Aux Heat" Displayed), BLANK-SCREEN (Blank/Dead Display — No Power), BLOWN-FUSE (HVAC Control Board Fuse Blown), COOL-ON-FLASH ("Cool On" Flashing on Display), and 3 more.

Error code NO-COOL also appears on 7 other Honeywell models: RTH7600D 7-Day Programmable Thermostat with Backlit Display, RTH9585WF Wi-Fi Color Touchscreen 7-Day Programmable Thermostat, T10 Pro Smart Thermostat with RedLINK Room Sensor, T6 Pro Programmable Thermostat (TH6320WF / TH6220U), TH4210U2002 PRO 4000 5-2 Day Programmable Thermostat, TH6220D1028 FocusPRO 6000 2H/2C Programmable Thermostat, and 1 more. The fix may vary by model, so check each model's specific page for tailored repair steps.