F3 error; outdoor unit may not start or may shut down frequently

Safety first: Turn off power at the outdoor isolator before opening the electrical compartment.

• 1. Locate T4 sensor: Typically mounted near the outdoor coil or on the condenser frame, exposed to ambient air.
• 2. Inspect physical condition: Check for broken sensor head, cracked insulation, or corrosion at the connector.
• 3. Measure resistance: Disconnect and test resistance vs. temperature. Replace if out of specification.
• 4. Secure placement: Ensure the new sensor is positioned where it can accurately sense outdoor air, not directly touching hot discharge lines.
• 5. Test operation: Restore power and run the unit in both COOL and HEAT (if heat pump) to confirm F3 does not return.