Thermostat shows E4 and cannot detect the outdoor unit or communicating furnace/air handler

Safety first: Turn off power to all HVAC components at the service disconnects and breaker panel before touching any wiring.

• Step 1 – Check wiring layout: Identify the iComfort communication bus (often labeled A/B, +/-, or Data/Com). Confirm that the same pair is daisy‑chained correctly from thermostat to indoor unit to outdoor unit or interface module.
• Step 2 – Inspect for damage: Look for staples through the cable, crushed sections, or splices that may have come loose. Repair or replace damaged cable sections.
• Step 3 – Verify polarity: Ensure the A wire is on A at all devices and B is on B (or + to +, - to -). Reversed polarity can cause E4.
• Step 4 – Check for shorts: With power off, disconnect the communication wires at the thermostat and equipment. Use a multimeter to check for continuity between the two bus conductors (there should be no direct short).
• Step 5 – Isolate devices: Reconnect the thermostat to only the indoor unit. Power up and see if E4 clears. Then add the outdoor unit. If E4 appears when a specific device is added, that device or its wiring is suspect.
• Step 6 – Replace faulty module: If wiring is correct and E4 persists, the communication circuit on the thermostat or equipment interface module may be defective and require replacement by a technician.