Medium Lockout
Fault 51 127 on Worcester Bosch Greenstar CDi Compact

Your boiler's control system has detected that the wrong type of temperature sensor has been installed or configured for your hot water system.

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

Cylinder temperature sensor type used as collector temperature sensor (TA).

What causes this fault?

This fault occurs when the boiler's internal computer detects a mismatch between the temperature sensor installed and the settings it is expecting to see. It most commonly happens if the wrong part was used during a recent repair or if the control board has developed a software glitch that misidentifies the hot water components.

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

A7View →E9View →

Related guides you might find useful

No hot water but heating works

Almost always a stuck diverter valve on combis.

Read →

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 →