High Lockout
F.65 on Vaillant ecoTEC exclusive

Your boiler’s internal computer brain is overheating, which has caused the system to shut down for safety.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Electronics temperature too high: Electronics too hot due to external effect, electronics defective

What causes this fault?

This fault occurs when the boiler's main control board reaches a temperature that risks melting internal components or causing a fire. The most common reasons are an internal electrical short circuit, poor ventilation around the boiler casing, or being located too close to an external heat source. If the heat is generated from within the board itself, it usually indicates that a specific electronic component has failed and is drawing too much current.

This fault is more common during heatwaves in the summer, especially if the boiler is installed in a poorly ventilated cupboard or a hot loft space.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

Related guides you might find useful