Medium Lockout
6C:0 to 6C:100 on Viessmann Vitodens 200-W

Your boiler's internal pump or fan is reporting an incorrect speed setting, which is usually a technical communication error between the internal computer and the parts that move water or air.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description

Set speed adjustable from 0 to 100%

What causes this fault?

This fault occurs when the boiler's control board sends a command to the pump or fan but receives a confused or incorrect speed signal in return. It is essentially a communication breakdown where the internal computer cannot confirm that these vital moving parts are spinning at the correct rate to safely manage heat and airflow.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

6C:100View →

Related guides you might find useful

Banging / kettling / whistling noise

Each noise points to a different fault.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Radiators cold at the bottom

Sludge build-up — needs flushing, not bleeding.

Read →