Thermostat shows E12 and indicates Y1/Y2 cooling output fault; outdoor unit may not start

Safety first: Turn off power to both indoor and outdoor units at their breakers before testing wiring.

Steps to troubleshoot E12 cooling output fault:

• 1. Check low-voltage fuse: Open the Goodman furnace/air handler panel and inspect the low-voltage fuse (often 3–5A automotive style). Replace if blown, but only after checking for shorts.
• 2. Inspect Y wiring: Verify that the Y1 (and Y2 if used) wires are securely connected at the thermostat and at the Goodman outdoor unit control board. Repair any damaged cable.
• 3. Test thermostat output: With power on and a call for cooling, measure 24VAC between Y1 and C at the furnace control board. If no voltage is present, the thermostat or its wiring is at fault.
• 4. Bypass thermostat: Temporarily jumper R to Y1 at the furnace control board. If the outdoor unit starts, the thermostat's Y1 output is likely defective.
• 5. Correct staging configuration: Ensure the thermostat is configured for the correct number of compressor stages to match the Goodman outdoor unit. Incorrect staging can trigger E12 on some models.
• 6. Replace thermostat if needed: If wiring and configuration are correct but the thermostat never energizes Y1/Y2, replace the thermostat.