Outdoor Heil air conditioner will not run; thermostat shows E1 communication error between indoor and outdoor unit

Safety first: Turn off power at the outdoor disconnect and the indoor air handler/furnace breaker before touching any wiring. Verify power is off with a non-contact voltage tester.

• Step 1 – Power reset: Turn off both indoor and outdoor breakers for 5 minutes, then restore power. Many Heil communicating systems will clear a transient E1 after a full power reset.
• Step 2 – Inspect thermostat wiring: Remove the thermostat and verify the low-voltage wires are firmly landed on the correct terminals (R, C, Y1, Y2, G, W). Look for loose strands, corrosion, or shorts between terminals.
• Step 3 – Check communication cable: At the indoor unit control board, locate the Y1/Com (or communication) terminals going to the outdoor unit. Confirm the same color wires land on the same terminals at both ends. Tighten any loose screws and repair any nicks or crushed sections of cable.
• Step 4 – Test continuity: With power still off, disconnect the communication pair at both ends and check continuity with a multimeter. Replace the cable if open or shorted.
• Step 5 – Inspect control boards: Look for burnt spots, swollen components, or corrosion on the indoor and outdoor control boards. If wiring is correct and the cable is good but E1 persists, the fault is likely a failed control board and should be replaced by a qualified technician.