BoilerFaultCodes 1.W1
Your boiler is simply displaying a status notification to indicate that the automatic weather-adjusting feature is currently active.
Worcester Bosch Greenstar Heatslave II Fault codes & diagnostics
50 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
OilCombi Oil 12-32 kW 2015-present
GC Numbers 47-440-4947-440-5047-440-51
The fault code is already on your display
Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →
Severity at a glance
Each cell = one fault code. Hover to identify. 19 High
3 Medium
28 Low
Just want the most common Worcester Bosch Greenstar Heatslave II faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes
All 50 documented codes
Tap any card for details
1.W2 Your boiler is struggling to read its external temperature sensor, which helps it adjust your heating based on the weather outside.
Your boiler is struggling to read its external temperature sensor, which helps it adjust your heating based on the weather outside.
Parts commonly replaced
- External weather compensation sensor
- Wiring harness
Related codes
1.W3 Your boiler is struggling to read the outdoor temperature sensor, so it is using a default setting to keep your home warm.
Your boiler is struggling to read the outdoor temperature sensor, so it is using a default setting to keep your home warm.
Parts commonly replaced
- External Temperature Sensor
- Wiring Loom
Related codes
1.W4
Your boiler has detected very cold temperatures and has automatically turned on your heating to protect the system from freezing.
2.7A Your boiler's blue light is displaying a status code that indicates a mismatch or error in your boiler's internal configuration settings.
Your boiler's blue light is displaying a status code that indicates a mismatch or error in your boiler's internal configuration settings.
Parts commonly replaced
- Main PCB
Related codes
9A Your boiler's computer module does not correctly match the specific model of your boiler, preventing it from starting up properly.
Your boiler's computer module does not correctly match the specific model of your boiler, preventing it from starting up properly.
Parts commonly replaced
- HCM (Heating Control Module)
- PCB (Printed Circuit Board)
Related codes
9F Your oil-fired boiler has stopped working because the burner failed to start, which can be caused by a fuel supply issue, a blocked filter, or the boiler needing a service.
Your oil-fired boiler has stopped working because the burner failed to start, which can be caused by a fuel supply issue, a blocked filter, or the boiler needing a service.
Parts commonly replaced
- Photocell
- Oil burner nozzle
- Oil filter
- Ignition electrodes
Related codes
9F855 Your boiler has stopped working because it cannot light the burner, which most often happens when your oil tank is empty or there is an issue with the oil supply.
Your boiler has stopped working because it cannot light the burner, which most often happens when your oil tank is empty or there is an issue with the oil supply.
What to check first
- Check the level of oil in your fuel tank to ensure it hasn't run dry.
- Verify that the oil supply valves to the boiler are fully open.
- Press the reset button on the control panel once to see if the burner restarts.
Parts commonly replaced
- Oil pump
- Photocell
- Oil nozzle
- Fuel filter
9F856 Your boiler has stopped working because it has been reset too many times while trying to clear a burner fault, and it has now locked itself for safety.
Your boiler has stopped working because it has been reset too many times while trying to clear a burner fault, and it has now locked itself for safety.
Parts commonly replaced
- Oil spray nozzle
- Ignition electrodes
- Oil pump
- Photocell
Related codes
200
Your boiler is simply working as intended to heat your radiators.
Showing 11–20 of 50
201
Your boiler is simply working normally to heat your hot water tank, rather than indicating a fault.
202
Your boiler is perfectly safe and is simply taking a short, programmed break to prevent it from turning on and off too frequently.
203 Your boiler is currently in standby mode and is working exactly as it should because there is no current request for heat or hot water.
Your boiler is currently in standby mode and is working exactly as it should because there is no current request for heat or hot water.
What to check first
- Check that your room thermostat or smart controller is set to a temperature higher than the current room temperature
- Ensure your heating timer or schedule is currently set to 'ON'
- Check that the hot water pre-heat function or your hot water schedule is active if you need an immediate hot water supply
218 Your boiler has likely experienced a glitch or internal hardware communication error within its main computer system.
Your boiler has likely experienced a glitch or internal hardware communication error within its main computer system.
What to check first
- Locate the main power switch for the boiler and turn it off.
- Wait for approximately 30 seconds.
- Turn the power back on and press the 'Reset' button on the boiler fascia for 3 to 5 seconds.
Parts commonly replaced
- Main Control Board (PCB)
- Wiring harness
270
Your boiler is simply performing a routine safety check as it starts up, which is a normal process that happens every time it is switched on.
358
Your boiler is automatically moving a internal valve to keep it from sticking, which is a normal maintenance feature rather than a fault.
380 Your solar water heating system has detected that the water inside your hot water tank is dangerously hot.
Your solar water heating system has detected that the water inside your hot water tank is dangerously hot.
Parts commonly replaced
- Solar tank temperature sensor
- Solar circulation pump
- Solar controller
A0 Your boiler is struggling to accurately measure the temperature of the water inside its internal storage tank, which means your hot water may not heat up correctly.
Your boiler is struggling to accurately measure the temperature of the water inside its internal storage tank, which means your hot water may not heat up correctly.
Parts commonly replaced
- Hot water thermal store sensor
- Wiring harness
A7 Your boiler is struggling to accurately measure the temperature of the water used for your taps and shower, which will likely prevent you from getting hot water.
Your boiler is struggling to accurately measure the temperature of the water used for your taps and shower, which will likely prevent you from getting hot water.
Parts commonly replaced
- DHW NTC Sensor
Alert Your boiler has detected an internal error and is waiting for you to look at the display to reveal the specific fault code causing the problem.
Your boiler has detected an internal error and is waiting for you to look at the display to reveal the specific fault code causing the problem.
Parts commonly replaced
- Printed Circuit Board (PCB)
- Internal Sensors
- Fan
Showing 21–30 of 50
BE Your boiler has identified a temporary internal issue that is preventing it from firing up, acting as a safety pause to protect the system.
Your boiler has identified a temporary internal issue that is preventing it from firing up, acting as a safety pause to protect the system.
Parts commonly replaced
- Ignition electrode
- Flame sensor
- Control board
D4 Your boiler has detected that the water is heating up unevenly inside the unit and has safely shut down to prevent damage.
Your boiler has detected that the water is heating up unevenly inside the unit and has safely shut down to prevent damage.
Parts commonly replaced
- NTC Temperature Sensor
- Central Heating Pump
- Plate Heat Exchanger
Related codes
E2 Your boiler is unable to safely detect if water is moving through the system, so it has shut down to prevent potential damage.
Your boiler is unable to safely detect if water is moving through the system, so it has shut down to prevent potential damage.
Parts commonly replaced
- Flow sensor
- Wiring harness
Related codes
E5 Your boiler has encountered an internal electronic communication error that is preventing it from running safely.
Your boiler has encountered an internal electronic communication error that is preventing it from running safely.
What to check first
- Press the 'Reset' button on the control panel once.
- Wait up to 15 minutes for the boiler to restart.
- If the code persists, do not attempt further repairs and contact a Gas Safe engineer.
Parts commonly replaced
- Main Printed Circuit Board (PCB)
- Wiring harness
Related codes
E9 Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage.
Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage.
Parts commonly replaced
- High limit thermostat
- Primary flow switch
- Pump
- PCB
Related codes
ECO Your boiler is simply in energy-saving mode, which means it will not pre-heat your hot water in order to save on fuel costs.
Your boiler is simply in energy-saving mode, which means it will not pre-heat your hot water in order to save on fuel costs.
What to check first
- Locate the ECO button on the boiler control panel
- Press and hold the ECO button until the light turns off if you prefer instant hot water
- Leave the setting as it is if you wish to continue saving energy
F0 Your boiler's control system has detected a faulty electrical connection or signs of water leakage inside the main electronic housing, causing it to shut down for safety.
Your boiler's control system has detected a faulty electrical connection or signs of water leakage inside the main electronic housing, causing it to shut down for safety.
Parts commonly replaced
- Burner Control Unit
- Wiring Harness
- Seal Kit
F0 263 Your boiler has briefly lost electrical power and is now waiting for you to manually restart it to resume normal operation.
Your boiler has briefly lost electrical power and is now waiting for you to manually restart it to resume normal operation.
What to check first
- Locate the reset button on the front control panel of your Greenstar Heatslave II.
- Press and release the reset button firmly.
- Wait a few moments for the boiler to cycle through its startup sequence and check if the blue light stops flashing.
Related codes
FO Your boiler's computer is having trouble reading its internal sensors, which means it has safely shut down to prevent incorrect operation.
Your boiler's computer is having trouble reading its internal sensors, which means it has safely shut down to prevent incorrect operation.
Parts commonly replaced
- Printed Circuit Board (PCB)
Related codes
H03 Your boiler is struggling to read the temperature outside, which may cause it to run less efficiently than usual.
Your boiler is struggling to read the temperature outside, which may cause it to run less efficiently than usual.
Parts commonly replaced
- External weather sensor
- Wiring loom
Showing 31–40 of 50
H11 Your boiler has identified a minor fault with a sensor that monitors the incoming mains cold water, but your heating and hot water will continue to work absolutely fine.
Your boiler has identified a minor fault with a sensor that monitors the incoming mains cold water, but your heating and hot water will continue to work absolutely fine.
Parts commonly replaced
- Domestic Cold Water Sensor
i1
This code is not an actual fault; it is simply showing your boiler's current operating status or mode of activity while it is running normally.
i2
This code is simply a record showing the last technical error your boiler experienced; it is not currently reporting an active fault.
i3
Your boiler is simply displaying its current heating power limit and is not actually experiencing a fault or error.
i4
This code is not actually an error; it is simply showing you the current maximum heating power level for your hot water production.
i6 Your boiler is simply displaying the current speed at which the hot water is flowing through your taps, rather than indicating a fault.
Your boiler is simply displaying the current speed at which the hot water is flowing through your taps, rather than indicating a fault.
What to check first
- Press the 'Reset' button or the 'Menu' button to return the screen to the standard operating display
- Monitor if the display persists after closing all hot water taps
- If the boiler is otherwise working normally, no further action is required
i7
Your boiler is displaying its current target heating temperature rather than an error message, which is normal operation while the system is waiting for a signal to provide heat.
i9
The display is simply showing the current temperature of the water inside the boiler rather than indicating a fault.
i10 Your boiler is temporarily shutting down because it has detected that the internal water temperature has become dangerously high.
Your boiler is temporarily shutting down because it has detected that the internal water temperature has become dangerously high.
Parts commonly replaced
- Central Heating Pump
- Plate Heat Exchanger
- NTC Sensor
Related codes
i11
This code is simply an information display, not a fault, showing you the current temperature of the hot water flowing through your system.
Showing 41–50 of 50
i12 Your boiler is simply displaying the current target temperature setting for your hot water, rather than indicating a fault.
Your boiler is simply displaying the current target temperature setting for your hot water, rather than indicating a fault.
What to check first
- Recognise that this is an information message rather than an error code
- Rotate the hot water control dial on the front panel to return the display to normal mode
- No further action is required as the boiler is functioning correctly
i13
This is not an error code, but rather a live status display showing the current temperature of the internal water store inside your boiler.
i14
This code is simply showing you the current temperature of the incoming cold water pipe, rather than indicating there is an actual fault with your boiler.
i15 Your boiler is simply displaying the current outdoor temperature because a weather sensor is connected, which is a normal status reading rather than a fault.
Your boiler is simply displaying the current outdoor temperature because a weather sensor is connected, which is a normal status reading rather than a fault.
What to check first
- No action is required as this is a status notification, not an error.
- Check your boiler display manual if you wish to toggle the default display view back to the target room temperature.
i19 Your boiler is simply reporting the current status of your heating timer settings rather than indicating a technical fault.
Your boiler is simply reporting the current status of your heating timer settings rather than indicating a technical fault.
What to check first
- Check your external programmer or timer dial on the front of the boiler
- Adjust the settings to your desired schedule for central heating and hot water
- If the code persists despite settings being active, toggle the timer switch off and on to reset the display
Related codes
i20 Your boiler is displaying its internal software version, which indicates a computer communication error rather than a physical breakdown, but the unit will not operate while this message is present.
Your boiler is displaying its internal software version, which indicates a computer communication error rather than a physical breakdown, but the unit will not operate while this message is present.
Parts commonly replaced
- Main Control PCB
Related codes
i21 Your boiler is displaying its internal software version, which indicates a configuration error rather than a physical part failure.
Your boiler is displaying its internal software version, which indicates a configuration error rather than a physical part failure.
Parts commonly replaced
- Main Control Board
- HIS PCB
Related codes
i22 Your boiler is displaying a specific tracking code that identifies its power configuration, which usually appears after an internal control module has been replaced or reset.
Your boiler is displaying a specific tracking code that identifies its power configuration, which usually appears after an internal control module has been replaced or reset.
Parts commonly replaced
- Heatronic Control Module
Related codes
i23 Your boiler is simply displaying its internal software version information rather than reporting a functional problem.
Your boiler is simply displaying its internal software version information rather than reporting a functional problem.
What to check first
- Press the 'Reset' button on the boiler control panel once
- Wait for the display to return to the standard temperature reading
- If it persists, turn the boiler off at the fused spur for 30 seconds and switch it back on
No code Your boiler has shut down because it is overheating, likely because water is trapped inside it or the pressure is too low to circulate properly.
Your boiler has shut down because it is overheating, likely because water is trapped inside it or the pressure is too low to circulate properly.
Parts commonly replaced
- Central heating pump
- Diverter valve
- NTC sensors