Trane communicating thermostat shows 'Err 3'; furnace may shut down.

Steps to check:

• Compare temperatures: Compare displayed room temperature to an accurate thermometer. Large discrepancies indicate sensor issues.
• Inspect remote sensor wiring: If a remote indoor sensor is used, check its wiring for shorts or opens.
• Reset thermostat: Power cycle the thermostat and furnace.

When to call a professional: If Err 3 persists, the thermostat or sensor should be replaced by a technician.