E1 displayed on indoor unit, outdoor unit does not start, no cooling

Safety first: Shut OFF power at both the indoor air handler/furnace breaker and the outdoor condenser disconnect before touching wiring.

• Step 1 – Visual wiring check: At the outdoor unit, remove the service panel. Verify the low-voltage thermostat/control wires (typically Y, C, R, and communication terminals on Day & Night communicating systems) are tight and not corroded or chewed by rodents.
• Step 2 – Match terminals: Confirm that the same color wire lands on the same terminal designation at both indoor and outdoor boards (e.g., A to A, B to B, C to C on communicating models). Correct any mismatches.
• Step 3 – Inspect cable condition: Follow the communication cable along its route. Look for staples through the jacket, UV damage, or splices. Repair or replace damaged cable with proper thermostat/communication cable.
• Step 4 – Power cycle both units: After correcting wiring, power both indoor and outdoor units OFF for 5 minutes, then ON. Wait several minutes to see if communication re-establishes and E1 clears.
• When to call a pro: If wiring is intact and E1 remains, a failed indoor or outdoor control board is likely. Board diagnostics and replacement should be done by a qualified technician.