High Lockout
F.70 on Vaillant ecoMAX 635

Your boiler's electronic brains are essentially having a communication breakdown and don't recognize each other, causing the unit to stop working for safety.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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

Invalid device specific communication (ASIC and main board)

What causes this fault?

This fault occurs when the main control board and the display interface within the boiler fail to identify one another. The most common reason is a memory error or electronic failure after a power surge, though it can also be caused by loose wiring connections between the two boards.

This fault is often triggered during the winter months when power fluctuations or surges are more frequent during stormy weather.

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.

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Related fault codes

F.61View →F.62View →F.63View →F.64View →F.65View →F.67View →

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