The indoor unit displays F0, and the air conditioner will not start cooling or heating.

Safety first: Turn off the AC using the remote, then isolate power at the circuit breaker or disconnect switch before opening any covers.

• Access the indoor sensor: Remove the front fascia/filter cover of the indoor unit. Locate the small NTC sensor clipped near the indoor air intake or on the evaporator coil (often a small bead sensor with two thin wires).
• Check wiring and connector: Inspect the sensor wires from the NTC to the indoor PCB, typically located on the right side of the unit. Look for broken insulation, pulled wires, or a loose connector on the PCB. Reseat the connector firmly.
• Test the sensor (if you have a multimeter): Disconnect the sensor from the PCB and measure resistance. At approximately 25°C, most Vaillant NTCs typically measure 5–10 kΩ. If the reading is infinite (open circuit) or near 0 Ω (short circuit), the sensor is defective.
• Clean and re‑clip: If the sensor has fallen out of its clip or is not positioned correctly in the airflow, re‑clip it securely to the evaporator or intake grille so it senses room air correctly.
• Replace if faulty: If the resistance is out of range or the error code persists, replace the indoor ambient NTC sensor. Disconnect the old sensor, route the new sensor along the same path, secure it with cable ties, and reconnect to the PCB.
• Power up and test: Restore power, clear the error by cycling the unit off/on from the remote, and run in COOL mode to verify normal operation.

If F0 returns after sensor replacement, the indoor PCB sensor circuit may be damaged and should be inspected by a qualified Vaillant technician.