Johnson Starley Comet 80 Fault codes & diagnostics

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

Combi Natural Gas 24 kW Discontinued

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33 High 3 Medium
Just want the most common Johnson Starley Comet 80 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 36 documented codes

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12

Your boiler has stopped working because it has detected a general internal error and has locked itself out for safety.

High DIY-safe £100-350

What to check first

  1. Locate the reset button on the front control panel
  2. Press and hold the reset button for 3 to 5 seconds
  3. Wait for the boiler to restart and check if the error clears

Parts commonly replaced

  • Main Control Board (PCB)
  • Ignition Electrode
  • Gas Valve
Full explanation for 12
13

Your boiler has tried to light itself several times without success and has now locked itself out for safety.

High DIY-safe £100-£280

What to check first

  1. Check if your gas meter has credit and the emergency control valve is open
  2. Check if other gas appliances, like a cooker, are working correctly
  3. Press and hold the reset button for five seconds to see if the boiler restarts

Parts commonly replaced

  • Ignition electrode
  • Flame sensing probe
  • Gas valve

Related codes

Full explanation for 13
21

Your boiler's internal computer is having trouble reading signals from its sensors, which has caused the system to stop working as a safety precaution.

High Engineer only £180-450

Parts commonly replaced

  • Main Control PCB
  • Wiring Loom

Related codes

Full explanation for 21
25

Your boiler's internal computer has encountered a software data error and cannot process its instructions properly.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Display Interface Module

Related codes

Full explanation for 25
26

Your boiler is repeatedly failing to keep its flame lit during operation, causing it to shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead

Related codes

Full explanation for 26
30

Your boiler has detected an electrical fault with the internal sensor that monitors water temperature, causing the system to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • Flow Temperature Sensor (NTC Thermistor)
  • Wiring harness
  • Main Control Board

Related codes

Full explanation for 30
31

Your boiler's heating system thermometer is broken or disconnected, meaning it cannot tell how hot the water is to safely heat your home.

High Engineer only £110-190

Parts commonly replaced

  • Central Heating (CH) Flow NTC thermistor
  • Wiring harness connectors

Related codes

Full explanation for 31
32

Your boiler has detected a fault with the sensor that monitors hot water temperature, meaning it cannot safely provide hot water to your taps.

High Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) Thermistor
  • Wiring Harness

Related codes

Full explanation for 32
34

Your boiler has detected that the electricity supply coming into your home is too low for it to operate safely and has shut down to protect its components.

High Engineer only £100-350

Parts commonly replaced

  • Main Control Board
  • Voltage Regulator
Full explanation for 34
37

Your boiler has detected that the water pressure in the central heating system is too low to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the silver braided filling loop hoses underneath or near your boiler.
  2. Slowly open the valves on the filling loop until you hear water entering the system.
  3. Watch the pressure gauge and close the valves tightly once the needle reaches 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel re-pressurisation
  • Pressure sensor

Related codes

Full explanation for 37
Showing 11–20 of 36
40

Your boiler has detected that there is too much water pressure in the system, which can cause the safety valve to leak or damage internal components.

Medium DIY-safe £80-220

What to check first

  1. Check that the filling loop valves are fully closed and not letting in water
  2. Bleed water from a radiator using a bleed key to reduce the system pressure
  3. Monitor the pressure gauge to ensure it returns to between 1.0 and 1.5 bar
  4. Reset the boiler once the pressure is within the normal operating range

Parts commonly replaced

  • Filling loop valve
  • Pressure relief valve (PRV)
  • Expansion vessel

Related codes

Full explanation for 40
41

Your boiler has stopped working because it cannot detect any water pressure in the system.

High DIY-safe £0-120

What to check first

  1. Locate the pressure gauge on the front of the boiler or underneath on the pipework.
  2. If the needle is below 1 bar, locate the filling loop (the silver flexible hose with two small valves).
  3. Open both valves slowly to let water into the system until the gauge reaches 1.5 bar, then close them tightly.
  4. Turn the boiler off and back on to reset the fault code.

Parts commonly replaced

  • Filling loop
  • Water pressure switch
  • MMI Interface ribbon cable

Related codes

Full explanation for 41
45

Your boiler has detected an electrical fault with the internal sensor that monitors exhaust heat, causing the system to shut down for safety.

High Engineer only £140-£240

Parts commonly replaced

  • Flue Gas Temperature Sensor (NTC)
  • Wiring Harness
  • Main Control Board

Related codes

Full explanation for 45
46

Your boiler has stopped working because it can no longer monitor the temperature of the exhaust gases safely.

High Engineer only £120-210

Parts commonly replaced

  • Flue Gas Temperature Sensor (NTC)
  • Connecting Wiring Harness

Related codes

Full explanation for 46
47

Your boiler has stopped working because it can no longer detect the water pressure due to a broken internal connection.

High Engineer only £140-£260

Parts commonly replaced

  • Water pressure transducer
  • Wiring harness

Related codes

Full explanation for 47
E01

Your boiler has failed to light after several attempts and has locked itself out for safety reasons.

High DIY-safe £120-350

What to check first

  1. Check that your gas supply is turned on at the meter
  2. If you have a prepaid meter, ensure you have credit available
  3. Press and hold the 'Reset' button for 5 seconds to try restarting the boiler

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe

Related codes

Full explanation for E01
E02

Your boiler thinks there is a flame present when there shouldn't be, causing the system to shut down for safety.

High Engineer only £120-350

Parts commonly replaced

  • Flame rectification probe
  • Gas valve
  • Main PCB

Related codes

Full explanation for E02
E03

Your boiler has switched itself off because it has become too hot inside, often due to a blockage or a circulation problem.

High DIY-safe £90-280

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the system water pressure is between 1 and 1.5 bar
  3. Press the 'Reset' button on the control panel to see if the fault clears

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Main heat exchanger

Related codes

Full explanation for E03
E05

Your boiler has stopped working because the fan that helps remove waste gases is not spinning correctly or communicating with the system.

High Engineer only £250-£450

Parts commonly replaced

  • Fan Assembly
  • Air Pressure Switch
  • PCB

Related codes

Full explanation for E05
E08

Your boiler has failed to detect a flame and has safely shut down to prevent gas from entering your home without being burned.

High Engineer only £120-£350

Parts commonly replaced

  • Flame rectification electrode
  • Ignition lead
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E08
Showing 21–30 of 36
E09

Your boiler has stopped working because its internal control system is not receiving the correct signals from the gas valve or circulating pump.

High Engineer only £180-450

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for E09
E12

Your boiler's internal computer memory has encountered a digital error, meaning the system cannot load the settings it needs to operate safely.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Display PCB

Related codes

Full explanation for E12
E15

Your boiler has shut down because its internal temperature sensors are giving conflicting or inconsistent readings.

High Engineer only £120-220

Parts commonly replaced

  • Flow NTC thermistor
  • Return NTC thermistor
  • Wiring harness

Related codes

Full explanation for E15
E16, E17, E18

Your boiler has identified that its internal temperature sensors are stuck or giving an impossible reading, causing the system to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Sensor Wiring Harness
  • Main Control Board

Related codes

Full explanation for E16, E17, E18
E21

Your boiler's electronic control board can no longer correctly read data from its internal sensors, causing it to shut down for safety.

High Engineer only £180-350

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring harness

Related codes

Full explanation for E21
E33

Your boiler's internal sensor that monitors the temperature of the water coming back into the unit has developed a fault, causing the system to stop running as a safety precaution.

High Engineer only £120-£210

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Connecting wiring harness

Related codes

Full explanation for E33
E34

Your boiler has detected that the electricity supply coming into the unit is too low to operate safely, causing it to shut down.

High Engineer only £120-350

Parts commonly replaced

  • Main PCB
  • Voltage Regulator
  • Wiring Harness

Related codes

Full explanation for E34
E35

Your boiler's temperature sensor is reporting an error, meaning the system can't accurately measure how hot the water is as it leaves the unit.

High Engineer only £90-180

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness
  • Main Control Board
Full explanation for E35
F07

Your boiler has detected a problem with the sensor that monitors waste gases, causing the system to shut down for safety.

High Engineer only £140-£260

Parts commonly replaced

  • Flue Thermistor
  • Exhaust Temperature Sensor
  • Wiring Harness
Full explanation for F07
F13

Your boiler has shut itself down as a safety precaution after someone tried to reset it remotely too many times.

Medium DIY-safe £0-120

What to check first

  1. Locate the main power switch (fused spur) for the boiler.
  2. Turn the power off for at least 30 seconds.
  3. Turn the power back on and press the reset button on the boiler control panel once.

Parts commonly replaced

  • Interface PCB
  • External controls/Thermostat
Full explanation for F13
Showing 31–36 of 36
F37

Your boiler has detected that there is not enough water pressure in the system to operate safely, which has caused it to shut down.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (a flexible silver hose) underneath or near the boiler.
  2. Open the small valves on the filling loop slowly until you hear water entering the system.
  3. Watch the pressure gauge and close the valves when the needle reaches between 1.0 and 1.5 bar.
  4. Restart the boiler to clear the fault code.

Parts commonly replaced

  • External filling loop
  • Expansion vessel
  • Pressure sensor
Full explanation for F37
F40

Your boiler has shut down because there is too much water pressure in the system, which can put unnecessary strain on the internal seals and pipework.

High DIY-safe £80-180

What to check first

  1. Check the filling loop handles are fully closed
  2. Bleed water from a radiator using a bleed key to reduce pressure
  3. Monitor the pressure gauge until it returns to between 1 and 1.5 bar
  4. Press the reset button if the fault remains displayed

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure relief valve
Full explanation for F40
F47

Your boiler has lost communication with the device that checks its water levels, meaning it cannot confirm if it is safe to run.

High Engineer only £120-220

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness
  • Main PCB
Full explanation for F47
F52

Your boiler is struggling to detect the temperature of your water, which means you may have inconsistent or no hot water from your taps.

Medium Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC Sensor)
  • Connecting wiring harness
Full explanation for F52
F53

Your boiler has switched itself off because it detected that the exhaust gases are getting too hot, which is a safety precaution to prevent damage.

High Engineer only £120-£250

Parts commonly replaced

  • Flue Thermistor / NTC Sensor
  • Main Heat Exchanger cleaning
  • PCB

Related codes

Full explanation for F53
Fault Image

Your boiler has detected an internal logic error or a critical component failure that prevents it from starting up safely.

High Engineer only £150-350

Parts commonly replaced

  • Main PCB
  • Control Board
Full explanation for Fault Image