High Lockout
Fault 02 / 100 on Worcester Bosch Greenstar CDi Compact

Your boiler's main control unit is having a communication breakdown with its internal safety components, meaning it cannot operate safely and has shut down.

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

Internal fault: ISM not responding.

What causes this fault?

This fault occurs when the boiler's central brain loses its connection with the internal safety module that monitors the flame. The most common cause is an electronic failure within the internal circuitry or a loose connection on the main control board. Because the boiler can no longer verify that it is burning gas safely, it shuts down instantly as a protective measure.

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

EAView →224View →C6View →

Related guides you might find useful

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 →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →