Thermostat displays E13 and will not energize compressor (Y output)

Safety: Turn off both indoor and outdoor unit breakers before working on wiring.

• Inspect Y wiring at thermostat: Remove the Lux thermostat and check the Y wire for damage or shorts to other conductors.
• Isolate Y circuit: Disconnect the Y wire at the thermostat, restore power, and see if E13 clears. If it does, the issue is downstream.
• Check outdoor unit: At the condenser, inspect the contactor coil wiring connected to Y and C. Look for burned insulation or insect damage.
• Measure coil resistance: With power off, measure resistance across the contactor coil. A very low resistance may indicate a shorted coil that can overload the thermostat output.
• Repair/replace contactor: Replace the contactor if the coil is shorted or physically damaged.
• Replace thermostat: If E13 persists with Y disconnected, the internal Y relay is likely failed and the thermostat should be replaced.