Thermostat displays E1 and will not control heating or cooling

Safety warning: Shut off power to the indoor unit at the breaker and service switch before opening any panels.

• Step 1 – Confirm indoor unit power: Verify the air handler/furnace breaker is ON and the blower door is properly closed so the door safety switch is engaged.
• Step 2 – Inspect thermostat to indoor wiring: Remove the thermostat from the wall and confirm the communication/data wires (often labeled D+, D−, A/B, or similar on ComfortLink II) are secure and not shorted.
• Step 3 – Inspect wiring at indoor control board: Remove the blower door, locate the low-voltage terminal strip or ComfortLink II communication terminals, and ensure all wires are tight and in the correct positions per the Trane wiring diagram on the panel.
• Step 4 – Look for corrosion or damage: Check for water damage, corrosion, or rodent damage on the control board and wiring harness. Repair or replace damaged wiring.
• Step 5 – Power cycle system: Turn power off for 5 minutes, then back on to allow the thermostat and indoor board to re-sync. Wait several minutes to see if E1 clears.
• Step 6 – If E1 remains: The indoor control board or thermostat communication circuitry may be faulty. At this point, professional testing of the board and bus voltages is recommended.