High Lockout
F.49 on Vaillant ecoMAX

Your boiler's internal communication system is experiencing a power drop, meaning the main control board cannot talk to your thermostat or other smart controls.

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

eBUS under voltage fault

What causes this fault?

This fault occurs when the electrical supply to the boiler's communication system drops below a safe level, preventing the internal computer from sending instructions to your thermostat. The most common reason is a short circuit in the low-voltage wiring or a component failure within a connected smart controller that is draining too much power. In some cases, the main circuit board itself may be failing to output the correct voltage required to run these external parts.

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 →

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