F5 is displayed, the unit may stop or run with reduced capacity, and defrost issues may occur in heat pump mode.

Safety: Power off the unit at the outdoor isolator. The discharge line can be extremely hot; allow sufficient time for it to cool before touching.

• Locate discharge sensor: Open the outdoor service panel. The discharge sensor is usually strapped to the small-diameter copper pipe exiting the compressor.
• Check attachment: Confirm the sensor is tightly clamped to the pipe with its metal tip in full thermal contact. Re‑secure it with a metal clip or high-temperature tape if loose.
• Inspect for overheating signs: Look for discoloration or burnt insulation on the discharge pipe and sensor cable, which may indicate over-temperature operation.
• Measure resistance: With the sensor unplugged and cooled to ambient, measure resistance and compare the reading to the NTC chart. Replace if the reading is out of range (e.g., open circuit, short circuit).
• Replace sensor: Install a new discharge pipe NTC sensor, ensuring it is firmly attached and its cable is routed away from moving parts.
• If F5 persists: Overheating may be due to low refrigerant charge, a blocked outdoor coil, or compressor issues. Clean the outdoor coil thoroughly and call a certified Vaillant technician for refrigerant and compressor diagnostics.