Thermostat displays E23 and indicates cooling output fault

Safety first: Turn off power to both indoor and outdoor units at the breakers before working on wiring.

Steps to troubleshoot:

• 1. Test cooling call: Lower the setpoint several degrees below room temperature and see if the outdoor unit starts. If not, E23 is likely valid.
• 2. Inspect Y wire at thermostat: Remove the thermostat and verify the Y/Y1 wire is secure and not contacting other terminals.
• 3. Inspect Y wire at furnace/air handler: Confirm the Y conductor is attached to the Y terminal on the control board and that the board’s fuse is intact.
• 4. Inspect at outdoor unit (advanced): With power off, open the outdoor unit control panel and verify the thermostat cable is intact and connected to the contactor coil terminals.
• 5. Test with jumper: With thermostat removed and power on, briefly jumper R to Y at the furnace board. If the outdoor unit starts, the thermostat Y relay is suspect.
• 6. Replace thermostat if Y relay is faulty: Replace the Emerson thermostat with a compatible model.

When to call a professional: If the outdoor unit does not start even with a direct R–Y jumper, an HVAC technician should check the contactor, low-pressure switches, and control board.