Thermostat displays E04, and cooling will not start; the AC condenser does not engage when cooling is requested.

Safety first: Shut off power to both the indoor air handler and outdoor condenser at their respective breakers before moving any wires.

• Step 1 – Verify thermostat Y connection: Remove the Geo thermostat and confirm the Y (or Y1) wire is fully seated and clamped in the Y terminal. Tighten the screw if needed.
• Step 2 – Check at air handler: At the indoor unit control board, verify the Y wire is secure on the Y terminal and that the cable jacket is intact.
• Step 3 – Continuity test: With power off and Y disconnected at both ends, test continuity of the Y conductor with a multimeter. Replace or repurpose a spare conductor if the wire is open.
• Step 4 – Test system with jumper: With power on, briefly jumper R to Y at the furnace/air handler control board. If the outdoor unit starts, the AC system is functional, and the fault lies in the thermostat or wiring.
• Step 5 – Inspect for shorts: Look for any place where the thermostat cable may be pinched or contacting sharp metal, especially near the air handler cabinet and wall penetration.
• Step 6 – Replace thermostat if needed: If wiring is good and the system runs with an R‑Y jumper, the Y relay on the Geo thermostat is likely defective. Replace the thermostat head unit.