Worcester Bosch 28i Junior Fault codes & diagnostics

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

Combi Natural Gas 28 kW 2006-2013 Discontinued

Replaced by: Greenstar 29i Junior

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.
9 High 1 Medium 6 Low
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 16 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
Blue indicator light off

Your boiler has no power and is completely switched off, meaning you have no heating or hot water until the electrical supply is restored.

High DIY-safe £0-120

What to check first

  1. Check that the main boiler power switch (usually a fused spur near the boiler) is turned to the ON position.
  2. Ensure the master dial on the boiler control panel is not set to the 'Off' position.
  3. Check your home's main consumer unit (fuse box) to see if a circuit breaker has tripped or a fuse has blown.

Parts commonly replaced

  • 3A Fuse
  • Internal PCB Fuse
  • Printed Circuit Board (PCB)
Full explanation for Blue indicator light off
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
Fast Flashing

Your boiler has detected an internal computer error that prevents it from starting, meaning you currently have no heating or hot water.

High Engineer only £250-400

Parts commonly replaced

  • Printed Circuit Board (PCB)
Full explanation for Fast Flashing
Digital codes
2 pulses / No light

Your boiler has been accidentally put into a technician's test mode and just needs to be switched back to its normal setting.

Low DIY-safe

What to check first

  1. Locate the small button with a 'spanner' icon or 'Service' label on the control panel.
  2. Press and hold the button for a few seconds until the flashing light stops or changes.
  3. The boiler should return to normal operation and provide heating and hot water as usual.
Full explanation for 2 pulses / No light
2 pulses No light

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.

Low DIY-safe

What to check first

  1. Locate the service button on the boiler control panel.
  2. Press the service button once to exit the current mode.
  3. Wait a few moments for the boiler to return to its standard operation status.
Full explanation for 2 pulses No light
5 pulses / No light

Your boiler has been manually switched into its maximum power testing mode and simply needs to be toggled back to normal operation.

Low DIY-safe

What to check first

  1. Locate the small button marked with a 'spanner' icon or 'service' label on the control panel.
  2. Press and hold the button for at least 5 seconds.
  3. The flashing light should stop, indicating the boiler has returned to its normal heating mode.
Full explanation for 5 pulses / No light
5 pulses No light

Your boiler is currently stuck in a special testing mode used by engineers rather than a fault condition.

Low DIY-safe

What to check first

  1. Locate the service button on the boiler control panel
  2. Press the service button once to exit the testing mode
  3. Ensure the boiler returns to normal operation with the flashing indicator light now solid or off as per usual
Full explanation for 5 pulses No light
Blue light off

Your boiler has lost all electrical power, meaning it cannot provide heating or hot water until the connection is restored.

High DIY-safe £90-250

What to check first

  1. Check your home's main fuse box or consumer unit to see if a trip switch has turned off.
  2. Ensure the fused spur switch located near the boiler is turned on.
  3. Check if other electrical appliances on the same circuit are working.

Parts commonly replaced

  • Main circuit board (PCB)
  • Internal fuse
  • Power cable
Full explanation for Blue light off
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 (No light)

Your boiler is not receiving electrical power or does not realise you are asking for heating or hot water.

High DIY-safe £80-250

What to check first

  1. Check if the boiler's power switch is turned on
  2. Check your home's consumer unit (fuse box) to see if a circuit has tripped
  3. Ensure your thermostat is turned up and has working batteries
  4. Check if the external timer or programmer is set to 'On'

Parts commonly replaced

  • Internal Fuse
  • Printed Circuit Board (PCB)
  • External Thermostat
Full explanation for Off (No light)
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