Common problems guide

The most common Worcester Bosch Greenstar 24i / 28i Junior problems

The faults most likely to send a Worcester Bosch Greenstar 24i / 28i Junior into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
15 documented codes
6 most common
1 DIY-safe checks
5 engineer needed

The 6 most common faults

01
On / Off Medium DIY-safe

Your boiler is providing hot water as normal, but there is a communication issue preventing the heating from turning on when you use your thermostat.

Why it happens: This issue occurs when the link between your heating controls and the boiler is broken, meaning the boiler doesn't 'hear' the signal to turn on. The most common reason is a pair of flat batteries in your wall thermostat or a technical failure in the wireless receiver box attached to the boiler.

What to try first

Check that the batteries in your wall thermostat or room programmer haven't run outEnsure the heating timer is set to 'ON' or 'AUTO' and the thermostat is turned up higher than the current room temperatureCheck if any wireless receiver attached to the boiler has a power light or is showing a signal connection

Est. cost
£60-250
Parts
Room Thermostat, Wireless Receiver, Diverter Valve Actuator
Full guide for On / Off
02
Off / Off High Engineer

Your boiler has lost all power and is completely unresponsive, meaning it is currently unable to provide any heating or hot water.

Est. cost
£150-350
Parts
Internal Fuses, Transformer, Control Board (PCB)
Full guide for Off / Off
03
Fast flash / Off High Engineer

Your boiler has detected a critical internal error and purposefully shut itself down to prevent damage, meaning you currently have no heating or hot water.

Est. cost
£150-300
Parts
NTC Temperature sensor, Fan unit, Wiring harness
Full guide for Fast flash / Off
04
Fast flash / Flashing High Engineer

Your boiler's internal computer has developed a fault and can no longer safely control the heating or hot water functions.

Est. cost
£300-500
Parts
Control board, Printed Circuit Board (PCB)
Full guide for Fast flash / Flashing
05
ERROR High Engineer

Your boiler's electronic brain is missing a crucial identification chip that tells it how to operate, so it has safely shut itself down until the part is properly installed.

Est. cost
£100-200
Parts
Code plug, PCB (Control Board)
Full guide for ERROR
06
No light High Engineer

Your boiler has completely lost power and is not showing any lights or signs of life.

Est. cost
£150-350
Parts
Main Control Board, Transformer, Fuses
Full guide for No light
Parts most often replaced

on the Worcester Bosch Greenstar 24i / 28i Junior

Room ThermostatWireless ReceiverDiverter Valve ActuatorInternal FusesTransformerControl Board (PCB)

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Worcester Bosch shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 15-code list for the Worcester Bosch Greenstar 24i / 28i Junior

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →