Carrier
No System Response to Thermostat Calls
Thermostat is on, but HVAC system will not start in any mode (heat, cool, or fan).
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Open R circuit between thermostat and control board, Failed Carrier control board input section, Incorrect system type configuration in thermostat, Safety switch (door, float switch) open preventing operation
How to Fix / Troubleshooting
Safety first: Turn off power to the HVAC system before accessing internal components.
- Step 1 – Confirm settings: Ensure the thermostat is set to Heat or Cool with a setpoint above/below room temperature and that the system is not in Off mode.
- Step 2 – Check equipment safety switches: Verify the furnace door is properly closed and any condensate float switches are not tripped (check for a full drain pan).
- Step 3 – Inspect R wire continuity: Ensure the R wire is secure at both thermostat and control board. If loose or broken, repair or replace.
- Step 4 – Bypass thermostat (advanced): With power off, remove thermostat wires from the control board and temporarily jumper R to G (fan), R to W (heat), or R to Y (cool). Restore power briefly to see if equipment starts. If it does, the thermostat or its wiring is at fault; if not, the control board or equipment is faulty.
- When to call a pro: If bypassing the thermostat does not start the system, a Carrier technician should test the control board and equipment.
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Repair Difficulty
Hard
4/5
Required Part
Carrier Furnace/Air Handler Control Board
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