Thermostat displays E1 and will not communicate with the indoor unit; no temperature or system status updates.

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

Steps to troubleshoot:

• 1. Power cycle the system: Turn off HVAC breakers for 2–3 minutes, then restore power. Check if E1 clears after reboot.
• 2. Inspect ABCD wiring at the thermostat: Remove the thermostat from the wall plate. Verify the four-conductor Bryant communication cable is firmly landed on the ABCD terminals in the correct order and color code (A to A, B to B, etc.). Tighten any loose screws.
• 3. Inspect ABCD wiring at the indoor unit control board: At the furnace/air handler, locate the Bryant Evolution/Connex control board. Confirm the same four conductors are securely connected to the ABCD terminals and match the thermostat end. Correct any reversed or loose wires.
• 4. Check for cable damage: Follow the cable run between thermostat and indoor unit. Look for staples through the cable, crushed sections, rodent damage, or splices. Repair or replace damaged sections using proper low-voltage splicing methods.
• 5. Isolate the thermostat: If possible, temporarily connect a short known-good 4-conductor cable directly from the thermostat to the indoor unit. If E1 clears, the original cable is faulty. If E1 persists, the thermostat or control board may be defective.
• 6. When to call a professional: If wiring is intact and correctly landed but E1 remains, a technician should test the Bryant Evolution/Connex control board and thermostat communication circuits and update firmware if needed.