High
3.3 on Worcester Bosch Greenstar 25i (ZWB 7-25)

Your boiler is failing to detect the flame needed to heat your water because the internal sensor is dirty or worn out.

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

Check ionisation current, Service Function 3.3. If the display shows 0 or 1, the electrode assembly must be cleaned or replaced.

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 guides you might find useful

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

When to replace your boiler

7 signs repair isn't worth it anymore.

Read →

Is boiler cover worth it?

Break-even maths on monthly cover.

Read →