Smart Thermostat Not Turning on AC
4.5
The thermostat says "Cooling" but the AC unit outside doesn't start.
Symptoms
- No cold air
- AC unit silent
- Thermostat error code
Possible Causes
- Blown C-wire fuse
- Incompatible wiring
- Relay failure
Step-by-Step Fix
1
Check HVAC Fuse
Check the 3A or 5A fuse on the control board inside your furnace/air handler.
2
Verify Wiring
Ensure the Y (Cooling) and G (Fan) wires are securely connected.
3
Test with Old Thermostat
If the old one works, the smart thermostat is likely faulty or misconfigured.
When to See a Technician
Smart thermostats require a "C" (Common) wire for consistent power.
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