F2 displayed, unit stops or runs only briefly, poor temperature control

Safety: Disconnect mains power before opening the indoor unit.

• Find the pipe sensor: Remove the front cover and right‑side electrical cover. The pipe sensor is usually strapped to a copper refrigerant pipe near the expansion device or coil outlet.
• Check attachment: Ensure the sensor is tightly strapped to the pipe with its metal tip in full contact. Re‑secure with new cable ties or thermal tape if loose.
• Inspect cable: Look for crushed or cut insulation where the pipe sensor cable passes through metal brackets. Repair minor insulation damage with high‑quality electrical tape; replace the sensor if conductors are damaged.
• Test sensor: Unplug from the PCB and measure resistance. Compare to Vaillant’s NTC chart. Replace if open, shorted, or clearly out of range.
• Install replacement: Fit the new indoor pipe NTC sensor in the same location, firmly strapped to the copper pipe, and route the cable along the original path.
• Restart: Restore power, clear the error by cycling power from the remote, and verify stable operation in both COOL and HEAT modes.