Indoor unit shows F3; system may shut down or defrost improperly in heat mode.

Safety first: Turn off power at the outdoor disconnect.

Steps:

• Locate T3 sensor: The outdoor coil temperature sensor is clipped to the outdoor condenser coil tubing.
• Inspect sensor and wires: Check for broken wires, corrosion, or a sensor that has fallen off the coil.
• Reconnect or reclip: If the sensor is loose, reattach it firmly to the coil tube.

Repair: If the sensor appears damaged or F3 persists, replace the T3 outdoor coil sensor. This is a simple part but requires safe access to the outdoor unit.