Medium Warning
48 on Viessmann Vitodens 333

Your boiler has lost communication with the temperature sensor that controls your second heating circuit, causing that specific zone to stop heating correctly.

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

Mixer closes - Heating circuit M2 flow temperature sensor lead break

What causes this fault?

This fault occurs when the boiler can no longer receive data from the sensor monitoring the temperature of your second heating zone, such as underfloor heating. The most common reasons are a loose connection in the internal wiring, a broken cable, or the sensor itself failing due to age.

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

40View →

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 →