Common problems guide

The most common Worcester Bosch Greenstar i Junior problems

The faults most likely to send a Worcester Bosch Greenstar i 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.
37 documented codes
6 most common
3 DIY-safe checks
3 engineer needed

The 6 most common faults

01
Blue light off High DIY-safe

Your boiler currently has no power, which means it cannot provide hot water or heating until the electrical connection is restored.

What to try first

Check that the main boiler power switch, the fused spur, and the thermostat are all switched on.Check your home's main fuse box (consumer unit) to see if a circuit breaker has tripped.If the boiler still has no power after checking your home's electrics, a qualified engineer should be called.

Est. cost
£100-250
Parts
Internal fuse, Printed Circuit Board (PCB), Transformer
Full guide for Blue light off
02
17 High DIY-safe

Your boiler has reached an unsafe temperature and has shut itself down to prevent damage.

What to try first

Check that all radiator valves are fully open to allow water to circulate.Ensure there is enough pressure in the system by checking the pressure gauge (should be between 1 and 1.5 bar).Press the 'Reset' button on the front of the boiler and hold it for five seconds.

Est. cost
£90-250
Parts
Overheat Thermostat (High Limit Stat), Central Heating Pump, Heat Exchanger (Flushing)
Full guide for 17
03
108 High DIY-safe

Your boiler has lost water pressure, which is preventing it from firing up to provide heat or hot water.

What to try first

Locate the filling loop (usually a silver braided hose under the boiler).Open the small black valves slowly until you hear water entering.Close the valves when the pressure gauge reaches between 1 and 1.5 bar.

Est. cost
£0-120
Parts
Filling loop, Expansion vessel, Pressure relief valve
Full guide for 108
04
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
05
On / Off (General fault) High Engineer

Your boiler has stopped responding to requests for heat or hot water and is stuck in a non-operational state.

Est. cost
£150-350
Parts
Fan, Printed Circuit Board (PCB)
Full guide for On / Off (General fault)
06
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
Parts most often replaced

on the Worcester Bosch Greenstar i Junior

Internal fusePrinted Circuit Board (PCB)TransformerOverheat Thermostat (High Limit Stat)Central Heating PumpHeat Exchanger (Flushing)

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 37-code list for the Worcester Bosch Greenstar i Junior

Related guides

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