Thermostat shows E2 and cannot control the outdoor unit; cooling or heating from heat pump does not start

Safety: Shut OFF power to the outdoor unit at the disconnect and breaker before opening the panel.

• Check outdoor power: Confirm the outdoor Ruud condenser/heat pump has line voltage (breaker ON, disconnect inserted). Look for any status LED on the control board.
• Inspect ABCD wiring: Remove the service panel and locate the ABCD terminals on the outdoor control board. Verify that the same wire colors used at the thermostat and indoor unit are correctly landed on A, B, C, and D. Tighten loose screws.
• Look for damage: Examine the low-voltage cable entering the outdoor unit for UV damage, cuts, or rodent chewing. Replace damaged cable with 18/5 thermostat wire rated for outdoor use.
• Reboot EcoNet network: After confirming wiring and power, restore power to the outdoor unit and indoor unit. Wait 5–10 minutes for the thermostat to rediscover the outdoor unit.
• Configuration check: On some Ruud systems, the thermostat must be configured for the correct outdoor unit type (AC vs heat pump, number of stages). Run the thermostat's equipment setup wizard and ensure the outdoor unit is correctly identified.
• Suspected board failure: If E2 persists and the outdoor unit never appears in the thermostat's equipment list, the outdoor control board's communication circuit may be faulty and should be replaced by a Ruud-certified technician.