High Lockout
F.64 on Vaillant Turbomax Plus 828 E

Your boiler has stopped working because its internal computer is receiving conflicting or faulty information from its temperature sensors.

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

Sensor or electronics fault | Check NTC's/connections or electronics defect

What causes this fault?

This fault occurs when the boiler's main control board detects a logic error between the heat sensors on the flow and return pipes. It usually happens because one sensor is sending a reading that is physically impossible or because the electrical signals are being interrupted by a faulty circuit board.

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

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

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →