F3
Vaillant
Outdoor Ambient Temperature Sensor Failure
F3 error, outdoor unit does not start or stops shortly after starting
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Open or shorted outdoor air NTC sensor, Sensor exposed or broken due to weather, Corroded connector at outdoor PCB, Faulty outdoor main PCB sensor circuit
How to Fix / Troubleshooting
Safety: Turn off the outdoor isolator or breaker. Wait several minutes for capacitors to discharge before touching the outdoor unit.
- Locate sensor: Remove the outdoor unit’s top or side service panel. The ambient sensor is usually a small NTC on two wires, mounted near the air intake grille or on the coil frame.
- Visual inspection: Check if the sensor body is cracked, UV‑damaged, or hanging loose. Ensure it is positioned in free air, not touching hot components.
- Check wiring and connector: Follow the sensor cable to the outdoor PCB. Clean any corrosion on the connector and reseat it firmly.
- Measure resistance: With the sensor unplugged, measure resistance and compare to the expected value for the current outdoor temperature. Replace if open, shorted, or far off spec.
- Replace sensor: Install a new outdoor ambient NTC sensor, mount it in the same location, and secure the cable away from fan blades and sharp edges.
- Test operation: Restore power and run the system. Confirm that F3 clears and that the unit responds correctly to outdoor temperature changes.
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Repair Difficulty
Medium
3/5
Required Part
Outdoor Ambient NTC Temperature Sensor
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