Trane communicating thermostat shows 'Err 4'; furnace may not operate correctly.

Steps to check:

• Inspect sensor location: Locate the Trane outdoor temperature sensor near the outdoor unit. Ensure it is not physically damaged.
• Check wiring: Verify sensor wires are intact and firmly connected at both sensor and furnace/thermostat terminals.
• Compare reading: If the thermostat displays an outdoor temperature far from actual, the sensor is likely faulty.

When to call a professional: Replacement of the outdoor sensor and verification of system configuration should be done by a technician.