F5 displayed, unit may stop or run with reduced capacity, defrost issues in heat pump mode

Safety: Power off at the outdoor isolator. The discharge line can be extremely hot; allow time 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 contact. Re‑secure 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 to the NTC chart. Replace if out of range.
• Replace sensor: Install a new discharge pipe NTC sensor, ensuring it is firmly attached and routed away from moving parts.
• If F5 persists: Overheating may be due to low refrigerant charge, blocked outdoor coil, or compressor issues. Clean the coil thoroughly and call a certified Vaillant technician for refrigerant and compressor diagnostics.