Worcester Bosch Greenstar 15sbi Fault codes & diagnostics

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

Regular Natural GasLPG 15 kW 2005-2013 Discontinued
GC Number 41-311-66

Replaced by: Greenstar 15Ri

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
22 High 3 Low
Just want the most common Worcester Bosch Greenstar 15sbi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 25 documented codes

Tap any card for details

LED / flash-pattern codes
Flashing light

Your boiler has failed to light the gas or cannot stay lit, meaning your heating and hot water have stopped working.

High DIY-safe £90-280

What to check first

  1. Check if your gas supply is active by testing another gas appliance like a hob.
  2. Ensure your credit is topped up if you have a pre-payment gas meter.
  3. Press the reset button on the boiler control panel for 5 seconds.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe
Full explanation for Flashing light
Lock-out (Flashing light)

Your boiler has stopped working because it cannot detect a flame to heat your water and home.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ionisation probe
  • Printed Circuit Board (PCB)
Full explanation for Lock-out (Flashing light)
Lock-out: Flashing light

Your boiler has stopped working because it cannot light the internal flame needed to heat your water or home.

High Engineer only £120-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board
Full explanation for Lock-out: Flashing light
Lockout light (flashing)

Your boiler has failed to light the flame to start the heating process and has safely shut itself down as a precaution.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board
Full explanation for Lockout light (flashing)
LOCKOUT LIGHT FLASHING

Your boiler has stopped working because it failed to light or stay lit, which has triggered a safety shutdown.

High DIY-safe £120-350

What to check first

  1. Check if your gas supply is active and credit is on your meter
  2. Press the reset button once to see if the boiler restarts
  3. Ensure any external gas valves are in the open position

Parts commonly replaced

  • Spark electrode
  • Flame sense electrode
  • Gas valve
  • Ignition lead
Full explanation for LOCKOUT LIGHT FLASHING
Digital codes
Autofroststat mode

Your boiler has detected very cold temperatures nearby and has temporarily turned itself on to prevent your pipes from freezing.

Low DIY-safe

What to check first

  1. Ensure your boiler is left switched on and connected to the power supply
  2. Check that your external condensate pipe is insulated to prevent freezing
  3. Ensure the room where the boiler is located is kept above freezing point
Full explanation for Autofroststat mode
Burner Lock-out

Your boiler has stopped working for safety reasons and needs a manual reset to try and restart the flame.

High DIY-safe £100-250

What to check first

  1. Locate the temperature control knob on the front panel.
  2. Turn the knob fully anticlockwise to the 'O' (Off/Reset) position.
  3. Wait 5 seconds, then turn the knob back to your desired temperature setting.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ionisation probe
Full explanation for Burner Lock-out
DEMAND LIGHT ONLY

Your boiler has reached its target temperature and is simply waiting for your radiators or hot water to cool down before heating up again.

Low DIY-safe

What to check first

  1. Check that your room thermostat is set high enough to call for heat
  2. Wait up to 10 minutes for the boiler to complete its anti-cycling delay
  3. Monitor the light to see if the burner fires once the water temperature drops

Parts commonly replaced

  • NTC Temperature Sensor
  • Control Board
Full explanation for DEMAND LIGHT ONLY
FAULT POINT A

Your boiler has lost all power and is completely unresponsive to your heating controls, meaning it cannot provide heat or hot water.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board
  • Internal Fuse
  • Transformer
Full explanation for FAULT POINT A
Showing 11–20 of 25
FAULT POINT C

Your boiler's heating system is failing to ignite unless the temperature knobs are turned all the way up, which indicates a problem with the internal sensors or control board.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • NTC Thermistor
  • Potentiometer
Full explanation for FAULT POINT C
FAULT POINT D

Your boiler has detected a critical internal error and has safely shut itself down to prevent damage, meaning your heating and hot water will not work until it is reset or repaired.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Ignition lead
  • Gas valve
Full explanation for FAULT POINT D
FAULT POINT E

Your boiler has detected a serious internal problem and has automatically shut down for safety reasons to prevent further damage.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board
  • Ignition Electrode
  • Gas Valve
Full explanation for FAULT POINT E
FAULT POINT F

Your boiler has identified an unusual temperature reading or a general internal disagreement, meaning it has safely shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • NTC sensor
  • Printed circuit board
  • Wiring harness
Full explanation for FAULT POINT F
lock-out

Your boiler has failed to ignite after several attempts and has switched itself off for safety reasons, meaning it cannot currently provide heat or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB
Full explanation for lock-out
Lock-out (Steady light)

Your boiler has detected that it is running too hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Pump
  • Diverter valve
  • Heat exchanger
  • NTC thermistor
Full explanation for Lock-out (Steady light)
Lock-out: Steady light

Your boiler has automatically turned itself off because it has become too hot to operate safely.

High Engineer only £150-350

Parts commonly replaced

  • Pump
  • Heat exchanger
  • NTC thermistor
  • Overheat thermostat
Full explanation for Lock-out: Steady light
Lockout light (constant)

Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage, while the pump keeps running to try and cool it back down.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Central heating pump
  • Diverter valve
  • Plate heat exchanger
Full explanation for Lockout light (constant)
Lockout light constantly on

Your boiler has switched itself off to prevent damage after detecting that it is getting too hot inside the unit.

High DIY-safe £120-280

What to check first

  1. Check the system pressure gauge and top up to 1.5 bar if low
  2. Ensure all radiator valves are fully open to allow water to flow
  3. Bleed any trapped air from your radiators
  4. Press the reset button after the boiler has cooled down

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Automatic Air Vent

Related codes

Full explanation for Lockout light constantly on
Showing 21–25 of 25
LOCKOUT LIGHT ON (CONSTANTLY)

Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Central heating pump
  • NTC sensor
Full explanation for LOCKOUT LIGHT ON (CONSTANTLY)
LOCKOUT LIGHT ON CONSTANTLY

Your boiler has switched itself off because it has detected an unsafe build-up of heat inside the unit.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Central heating pump
  • NTC sensor
Full explanation for LOCKOUT LIGHT ON CONSTANTLY
NO LIGHTS AT ALL

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

High Engineer only £120-250

Parts commonly replaced

  • Control board fuse
  • Main control board
  • Wiring harness
Full explanation for NO LIGHTS AT ALL
OTHER FAULTS

Your boiler has detected that the water inside is reaching temperatures that are either too high or too low for safe operation, causing the system to shut down.

High Engineer only £120-350

Parts commonly replaced

  • NTC Thermistor
  • Gas Valve
  • Printed Circuit Board (PCB)
  • Heat Exchanger

Related codes

Full explanation for OTHER FAULTS
Steady light

Your boiler has switched itself off because it has become too hot to operate safely.

High DIY-safe £90-180

What to check first

  1. Check that your radiator valves are open to allow water to flow.
  2. Wait for the boiler to cool down completely.
  3. Press the reset button once to see if the system restarts.

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Thermistor

Related codes

Full explanation for Steady light