Thermostat displays E40 and shows a message about incompatible equipment

Important: Trane communicating thermostats are designed for specific Trane systems. Using them on incompatible equipment can cause faults.

• Step 1 – Verify equipment type: Confirm that both indoor and outdoor units are Trane communicating models if using a ComfortLink II thermostat.
• Step 2 – Check wiring mode: If using the thermostat in conventional mode, ensure all wires are on standard terminals (R, C, Y, W, G, O/B) and not on communicating bus terminals.
• Step 3 – Reconfigure thermostat: In installer setup, select the correct system type (communicating vs conventional) and equipment stages.
• Step 4 – If still incompatible: You may need a different Trane thermostat model designed for conventional systems, or to upgrade equipment to a compatible communicating system.