High Lockout
b5 on Viessmann Vitodens

Your boiler has failed to complete its initial setup process, meaning it hasn't properly cleared air from the system or finished its first startup sequence.

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

Installation in unfinished state - Boiler venting Fault in burner / Ignition valve

What causes this fault?

This fault occurs when the boiler's automatic commissioning program fails to complete, often because the system hasn't been fully bled of air during setup. It signifies that the internal sensors cannot confirm a stable flame or safe operating conditions, causing the computer to stop the sequence for safety. The root cause is usually a snag during the initial 'purging' phase or a component like the ignition electrode failing to spark correctly.

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

b4View →

Related guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Pilot light keeps going out

Older boilers — thermocouple, draught, gas supply.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →