Gas Safe: Gas work, flue work, and PCB replacement on a gas boiler must be carried out by a Gas Safe registered engineer. Homeowners can check obvious items such as heating pressure, condensate overflow, and whether a control is unplugged — then call an engineer.
Always confirm the part number against the boiler GC number and the installation manual before ordering.
Overview
On VU/VUW boilers F.13 is a short on the warm-start or cylinder NTC. On VUW with actoSTOR it is a short on the store NTC and should appear with F.91.
Vaillant meaning: Short circuit warm-start / cylinder sensor (VU/VUW) or actoSTOR cylinder NTC with F.91
Possible causes
- NTC internally shorted
- Cable short to harness or casing
- Wrong sensor fitted or pins bridged by moisture
Possible solutions
- Identify whether the boiler is a standard VU/VUW or actoSTOR.
- Unplug the relevant NTC. If the fault clears, replace that NTC and inspect the lead.
- On actoSTOR, treat F.91 at the same time.
Possible parts needed
- Warm-start / DHW or cylinder NTC for the exact range (confirm GC)
- actoSTOR cylinder NTC where a store is fitted
- NTC cable / harness