Worcester Bosch 28i Junior Fault codes & diagnostics

7 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 28 kW 2006-2013 Discontinued
GC Numbers 47-311-8647-311-8747-311-9047-311-9147-406-56

Replaced by: Greenstar 29i Junior

Download the Worcester Bosch 28i Junior manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

This boiler uses indicator light combinations

Note which lights are on, off, or flashing and at what speed. Match the combination to the cards below. Do not reset until you have noted the pattern. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
6 High 1 Medium
Just want the most common Worcester Bosch 28i Junior faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 7 documented codes

Tap any card for details

LED / flash-pattern codes
Blue indicator light flashing

Your boiler is signalling a general internal fault that has caused it to stop working to ensure your safety.

High Engineer only £120-250

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Air Pressure Switch
  • Fan
Full explanation for Blue indicator light flashing
Fast flash

Your boiler has detected a safety-related interruption and has automatically shut down to prevent potential issues.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed circuit board (PCB)
  • Flame sensing probe
Full explanation for Fast flash
Fast flash / Flashing

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

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Control Box
Full explanation for Fast flash / Flashing
Fast flash / Off

Your boiler has shut itself down for safety because it has detected a significant electrical or mechanical issue with its internal components.

High DIY-safe £120-350

What to check first

  1. Press and hold the 'Reset' button for five seconds
  2. Wait for the indicator light to stop flashing or change to a steady light
  3. If the boiler refires, check that your thermostat is calling for heat

Parts commonly replaced

  • Temperature Sensor (NTC thermistor)
  • Fan Unit
  • Wiring Harness
  • PCB (Control Board)
Full explanation for Fast flash / Off
Digital codes
LOCKOUT

Your boiler has failed to light the flame after several attempts and has safely shut itself down to prevent a gas buildup.

High Engineer only £180-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for LOCKOUT
Off / Off

Your boiler has completely lost power and its display screen is blank, meaning it cannot operate or heat your home.

High DIY-safe £100-£450

What to check first

  1. Check if other appliances are working to rule out a general power cut
  2. Check your home's main consumer unit (fuse box) to see if a circuit has tripped
  3. Check the fused spur switch next to the boiler is turned on and the fuse inside hasn't blown

Parts commonly replaced

  • Internal PCB fuse
  • Printed Circuit Board (PCB)
  • External Fused Spur
Full explanation for Off / Off
On / Off

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.

Medium DIY-safe £60-250

What to check first

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

Parts commonly replaced

  • Room Thermostat
  • Wireless Receiver
  • Diverter Valve Actuator
Full explanation for On / Off
Status & diagnostic codes

10 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

Blue indicator light off Status Your boiler has no power and is completely switched off, meaning you have no heating or hot water until the electrical supply is restored. 2 pulses Diagnostic Your boiler is currently running in a restricted test setting, which means it is operating at a very low heat level rather than normal capacity. 2 pulses / No light Diagnostic Your boiler has been accidentally put into a technician's test mode and just needs to be switched back to its normal setting. 2 pulses No light Diagnostic Your boiler is currently in a manual testing mode used by engineers rather than broken, which is why it has stopped heating your home or water as usual. 5 pulses Diagnostic Your boiler is currently in a manual testing mode set to operate at its maximum output, rather than running a normal heating or hot water cycle. 5 pulses / No light Diagnostic Your boiler has been manually switched into its maximum power testing mode and simply needs to be toggled back to normal operation. 5 pulses No light Diagnostic Your boiler is currently stuck in a special testing mode used by engineers rather than a fault condition. Blue light on Status Your boiler is operating exactly as it should and there is nothing to worry about. Off (No light) Status Your boiler is not receiving electrical power or does not realise you are asking for heating or hot water. Steady on (Burner lit) Status Your boiler is working exactly as it should and is currently burning gas to provide heating or hot water.