Thermostat shows E6 and cannot communicate with the American Standard communicating HVAC system

Safety: Turn off power to both indoor and outdoor units at the breakers before touching any ABCD or data bus wiring.

• Verify wiring at thermostat: Remove the thermostat and confirm that the A, B, C, D (or similar) terminals are wired exactly as per American Standard communicating system diagrams. Colors must match at all devices.
• Check for shorts: Inspect the communication cable for staples through the jacket, crushed sections, or exposed copper. Repair or replace damaged cable.
• Confirm polarity: At the furnace/air handler and outdoor unit, verify that A connects to A, B to B, etc. Miswiring will cause E6.
• Isolate devices: Disconnect the outdoor unit from the bus and see if E6 clears when only the indoor unit and thermostat are connected. Then reconnect the outdoor unit. This helps identify which device is causing the fault.

When to call a pro: Diagnosing bus issues often requires a technician familiar with American Standard communicating systems and their specific voltage and signal characteristics.