Thermostat shows E10 and cannot communicate with Johnson Controls zoning panel or rooftop unit controller over the communication bus.

Safety: Turn off power to all connected controllers and the thermostat before working on communication wiring.

• Step 1 – Verify wiring: Check the communication terminals on the thermostat (e.g., + / - / COM or labeled BACnet/Modbus). Ensure wires are landed correctly and match the controller or zone panel terminals.
• Step 2 – Check polarity: On RS‑485 systems, ensure the + and - conductors are not reversed between devices.
• Step 3 – Inspect cable: Follow the communication cable run and look for kinks, cuts, or splices. Repair any damaged sections using proper twisted‑pair cable.
• Step 4 – Confirm addressing and baud rate: In the thermostat installer menu and on the connected controller, verify that network address, baud rate, and protocol settings match Johnson Controls specifications.
• Step 5 – Isolate devices: Disconnect all but one controller from the bus and test. A single faulty device can bring down the entire bus.
• Step 6 – Replace thermostat or controller transceiver: If wiring and settings are correct but E10 persists only when this thermostat is connected, the thermostat’s communication transceiver is likely failed. Replace the thermostat. If all thermostats fail to communicate, the issue may be in the zoning panel or rooftop unit controller.