Low Warning
91:2 on Viessmann Vitodens 333-F

Your boiler is experiencing a communication issue with the accessory that controls different temperature zones in your home, often caused by a temporary wiring hitch or a sensor problem.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description

External heating program changeover applies to heating circuit with mixer

What causes this fault?

This fault occurs when the boiler’s main control board loses the electronic signal from the module that manages different heating zones or underfloor heating. It is usually caused by a loose connection in the wiring between these components or a fault within the external mixer module itself, which prevents the boiler from knowing which temperature to send to specific parts of your home.

This issue is often noticed in autumn when homeowners switch their heating back on after summer and the external control modules fail to synchronise correctly.

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

91:1View →

Related guides you might find useful

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →