Safety: Switch off the AC and isolate power at the breaker before removing covers.

Locate the evaporator sensor: Remove the front cover and filters. Lift the plastic housing to expose the evaporator coil. The coil sensor is usually clipped into the fins or attached with a metal clip near the middle of the coil.
Inspect placement: Ensure the sensor is firmly clipped to the coil. If it is hanging loose, re‑clip it so it has good thermal contact with the fins.
Check wiring: Trace the sensor wires back to the indoor PCB. Look for pinched, cut, or burnt wires. Reseat the connector on the PCB.
Measure resistance: With the unit powered off and sensor unplugged, measure resistance. Compare to the ambient temperature chart in the Vaillant service manual (typically several kΩ at room temperature). Replace the sensor if the reading is open, shorted, or far out of spec.
Replace sensor: If defective, install a new indoor coil NTC sensor, routing the cable away from sharp metal edges and re‑securing with clips or cable ties.
Test: Restore power and run the unit in COOL mode. Confirm that F1 does not reappear and that the evaporator does not freeze.

Persistent F1 after sensor replacement indicates a likely indoor PCB fault that should be diagnosed by a professional.

Indoor unit shows F1 and stops operation shortly after startup

Possible Causes