Indoor unit displays F4 and system may not heat or cool correctly.

Safety first: Shut off power at the outdoor disconnect.

Steps:

• Locate T4 sensor: The outdoor ambient temperature sensor is usually mounted near the outdoor unit air intake, not on the coil.
• Check physical condition: Inspect for cracks, broken housing, or chewed wires.
• Verify connection: Ensure the sensor plug is fully seated on the outdoor PCB.

Repair: Replace the T4 outdoor ambient sensor if damaged or if F4 continues after reseating the connector.