F4 error, the outdoor unit runs briefly then shuts down, leading to poor heating performance.

Safety: Isolate power at the outdoor disconnect switch before removing any 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 condenser coil.
• Check mounting: Ensure the sensor is firmly clipped to the coil. If it has fallen off, re‑clip it securely or use a new metal clip or thermal tape to ensure good thermal contact.
• Inspect cable: Look for chafing where the sensor cable passes through metal. Secure the cable with grommets or tape to prevent future damage.
• Test sensor: Disconnect from the PCB and measure resistance. Compare the reading to Vaillant’s NTC chart. Replace if the readings are abnormal (e.g., open circuit, short circuit, or out of range).
• Replace sensor: Fit a new outdoor coil NTC sensor, route its 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 that F4 does not reappear.