F4
Vaillant
Outdoor Coil (Condenser) Temperature Sensor Failure
F4 error, outdoor unit runs briefly then shuts down, poor heating performance
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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
Failed outdoor coil NTC sensor, Sensor not in contact with coil, Damaged sensor harness from vibration, Outdoor PCB sensor input failure
How to Fix / Troubleshooting
Safety: Isolate power at the outdoor disconnect before removing panels.
- Access coil sensor: Remove the outdoor unit’s front or side panel to expose the condenser coil. The coil sensor is typically clipped to the coil fins or strapped to a refrigerant tube near the middle of the coil.
- Check mounting: Ensure the sensor is firmly clipped to the coil. If it has fallen off, re‑clip it or use a new metal clip or thermal tape to ensure good contact.
- Inspect cable: Look for chafing where the cable passes through metal. Secure with grommets or tape to prevent future damage.
- Test sensor: Disconnect from the PCB and measure resistance. Compare to Vaillant’s NTC chart. Replace if readings are abnormal.
- Replace sensor: Fit a new outdoor coil NTC sensor, route the cable neatly, and secure it with cable ties.
- Restart and monitor: Restore power and run the unit in HEAT mode. Monitor for stable operation and ensure F4 does not reappear.
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Repair Difficulty
Medium
3/5
Required Part
Outdoor Coil NTC Temperature Sensor
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