Johnson Starley Comet 60 Fault codes & diagnostics

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

Regular Natural Gas 17.6 kW Discontinued

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31 High 5 Medium
Just want the most common Johnson Starley Comet 60 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 entered a general safety shutdown and needs to be manually restarted to check if the error is temporary.

High DIY-safe £100-250

What to check first

  1. Locate the reset button on the boiler control panel
  2. Press and hold the reset button for several seconds
  3. Wait for the boiler to complete its startup sequence
  4. Ensure your thermostat is calling for heat

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Printed Circuit Board (PCB)
Full explanation for 12
13

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

High DIY-safe £100-280

What to check first

  1. Check that your gas meter has credit (if using a prepay meter).
  2. Ensure the gas isolation valve to the boiler is fully open.
  3. Press the 'Reset' button on the control panel to see if the boiler restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe

Related codes

Full explanation for 13
21

Your boiler's internal computer is having trouble processing signals from its sensors, preventing it from starting properly.

High Engineer only £180-450

Parts commonly replaced

  • Main Control Board (PCB)
  • Internal Wiring Harness
Full explanation for 21
25

Your boiler's internal computer has encountered a software data error and can no longer process its instructions correctly.

High Engineer only £250-450

Parts commonly replaced

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

Related codes

Full explanation for 25
26

Your boiler is struggling to keep its flame lit and has shut down as a safety precaution after failing several times in a row.

High Engineer only £120-350

Parts commonly replaced

  • Flame sense electrode
  • Gas valve
  • Detection lead
  • PCB

Related codes

Full explanation for 26
30

Your boiler has sensed a fault with a temperature sensor and has stopped running to prevent overheating.

High Engineer only £90-160

Parts commonly replaced

  • Flow Temperature Sensor (NTC Thermistor)
  • Wiring Harness

Related codes

Full explanation for 30
31

Your boiler has detected a problem with the internal temperature sensor that monitors your heating water, meaning it cannot safely regulate how hot the radiators get.

High Engineer only £120-190

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for 31
32

Your boiler has detected a software or electrical fault with the sensor that monitors your hot water temperature, which may prevent it from heating up.

Medium Engineer only £110-190

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 from your mains is too weak for it to operate safely.

Medium DIY-safe £0-120

What to check first

  1. Check if other appliances in your home are flickering or underperforming
  2. Switch the boiler off at the fused spur switch and then back on to reset the electronics
  3. Contact your energy network provider to check for local power fluctuations or brownouts

Parts commonly replaced

  • None (usually an external power grid issue)
  • Printed Circuit Board (PCB) if monitoring fails

Related codes

Full explanation for 34
37

Your boiler has stopped working because the water pressure in the central heating system has dropped too low.

High DIY-safe £0-120

What to check first

  1. Locate the external filling loop, which is usually a flexible silver hose with two small valves.
  2. Slowly open both valves to allow water into the system while watching the pressure gauge.
  3. Close both valves tightly once the gauge reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop hose
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for 37
Showing 11–20 of 36
40

Your boiler has detected that there is too much water pressure inside the system, causing it to shut down for safety.

High DIY-safe £0-120

What to check first

  1. Check that the silver filling loop valves are tightly closed
  2. Bleed water from a radiator using a bleed key to reduce pressure
  3. Monitor the pressure gauge to ensure it stays between 1.0 and 1.5 bar
  4. Reset the boiler once the pressure is back in the green zone

Parts commonly replaced

  • Filling loop valve
  • Pressure relief valve
  • 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 £60-180

What to check first

  1. Check the pressure gauge on the front of the boiler or under the casing.
  2. If the pressure is below 1.0 bar, locate the filling loop (braided flexible hose) underneath the boiler.
  3. Slowly open the valves on the filling loop to top up the water until the gauge reads between 1.0 and 1.5 bar.

Parts commonly replaced

  • Water pressure switch
  • Pressure sensor
  • Expansion vessel recharge

Related codes

Full explanation for 41
45

Your boiler has detected a fault with the sensor that monitors exhaust temperatures and has shut down as a safety precaution.

High Engineer only £120-£220

Parts commonly replaced

  • Flue gas temperature sensor (NTC)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for 45
46

Your boiler has lost connection with its internal thermometer that monitors exhaust fumes, causing the system to shut down for safety.

High Engineer only £120-£200

Parts commonly replaced

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

Related codes

Full explanation for 46
47

Your boiler has lost its connection to the sensor that monitors water levels, so it has shut down for safety.

High Engineer only £120-190

Parts commonly replaced

  • Water pressure sensor
  • Sensor wiring harness

Related codes

Full explanation for 47
E01

Your boiler has failed to light after several attempts, meaning it has shut down to prevent a safety issue.

High DIY-safe £100-280

What to check first

  1. Check if other gas appliances, like a cooker, are working to confirm your gas supply is active.
  2. Check that your pre-paid gas meter has credit, if applicable.
  3. Press the Reset button on the control panel to see if the boiler restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe

Related codes

Full explanation for E01
E02

Your boiler thinks it sees a flame even though it hasn't finished its start-up sequence, so it has shut down for safety.

High Engineer only £140-280

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Main PCB

Related codes

Full explanation for E02
E03

Your boiler has shut down because it has detected it is getting too hot inside the unit.

High DIY-safe £90-250

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure there is enough water pressure in the system (check the gauge)
  3. Once the boiler has cooled down, try pressing the reset button on the control panel

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Thermal Cut-out Switch

Related codes

Full explanation for E03
E05

Your boiler has stopped working because the internal fan that clears away waste gases is not spinning at the correct speed.

High Engineer only £180-350

Parts commonly replaced

  • Extractor fan motor
  • Air pressure switch
  • Main PCB

Related codes

Full explanation for E05
E08

Your boiler is failing to detect the flame needed to heat your water, causing the system to shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Flame rectification probe
  • Ignition electrode
  • PCB

Related codes

Full explanation for E08
Showing 21–30 of 36
E09

Your boiler has stopped working because its internal control board cannot confirm if a gas valve is opening or closing correctly.

High Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Main PCB (Control Board)
  • Wiring harness

Related codes

Full explanation for E09
E12

Your boiler's computer memory has encountered an internal error and has locked itself to prevent incorrect operation.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Display Interface Module
Full explanation for E12
E15

Your boiler has switched itself off because its internal sensors are reporting conflicting temperature readings.

High Engineer only £140-£260

Parts commonly replaced

  • Flow NTC thermistor
  • Return NTC thermistor
  • Main PCB

Related codes

Full explanation for E15
E16, E17, E18

Your boiler has detected that a temperature sensor is giving a constant, unchanging reading, causing the system to lock itself down for safety.

High Engineer only £120-£210

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Wiring harness

Related codes

Full explanation for E16, E17, E18
E21

Your boiler's electronic control board has detected an internal communication error and has locked itself out for safety.

High DIY-safe £180-450

What to check first

  1. Locate the main power switch for the boiler
  2. Turn the power off and wait for 30 seconds
  3. Turn the power back on and press the 'Reset' button on the display

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Loom
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, preventing it from heating correctly.

High Engineer only £120-190

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Connecting wiring harness
Full explanation for E33
E34

Your boiler has shut down because it is not receiving enough electrical voltage to operate safely and correctly.

High DIY-safe £90-250

What to check first

  1. Check if there is a regional power cut or a dip in local grid voltage
  2. Ensure the boiler's fused spur switch is fully on and the fuse hasn't partially blown
  3. Check if other high-power appliances are causing a voltage drop on your home circuit

Parts commonly replaced

  • External Voltage Stabiliser
  • Main Control PCB
  • Electric Heating Element

Related codes

Full explanation for E34
E35

Your boiler has detected a problem with the internal sensor that monitors the temperature of the water leaving the unit, causing it to stop heating.

High Engineer only £110-190

Parts commonly replaced

  • Flow Thermistor (NTC sensor)
  • Connecting wiring harness
Full explanation for E35
F07

Your boiler has detected a problem with the sensor that monitors internal temperatures, causing the system to shut down as a safety precaution.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness
  • Main Control Board
Full explanation for F07
F13

Your boiler has stopped working because too many remote reset attempts have occurred, causing the system to lock itself for safety.

Medium DIY-safe £0-120

What to check first

  1. Locate the main power switch for the boiler
  2. Turn the power off at the fused spur for 30 seconds
  3. Turn the power back on to hard-reset the control board

Parts commonly replaced

  • Main Control Board
  • External Thermostat/Controller
Full explanation for F13
Showing 31–36 of 36
F37

Your boiler has detected that the water pressure is too low, which prevents it from heating properly.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop, which is usually a silver flexible hose underneath the boiler.
  2. Slowly open the one or two taps on the hose until you hear water moving.
  3. Watch the pressure gauge and close the taps once it reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor
Full explanation for F37
F40

Your boiler has detected that there is too much water pressure in the system, which can put unnecessary stress on the internal pipes and seals.

Medium DIY-safe £80-180

What to check first

  1. Check that the filling loop valves are fully closed
  2. Bleed a small amount of water from a radiator using a bleed key
  3. Monitor the pressure gauge to ensure it returns to between 1.0 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 (PRV)
Full explanation for F40
F47

Your boiler has lost its connection to the sensor that monitors water pressure and has shut down as a safety precaution.

High Engineer only £120-190

Parts commonly replaced

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

Your boiler is having trouble measuring the temperature of your hot water, which usually means it cannot heat water for your taps correctly.

Medium Engineer only £90-160

Parts commonly replaced

  • DHW thermistor
  • Connecting wiring harness
Full explanation for F52
F53

Your boiler has switched itself off because the waste gases are too hot, which is a safety measure to protect the internal components.

High Engineer only £120-£240

Parts commonly replaced

  • Flue thermistor
  • Heat exchanger
  • Main PCB

Related codes

Full explanation for F53
Fault Image

Your boiler has detected that it is not working correctly and has shut down as a safety precaution.

High DIY-safe £150-350

What to check first

  1. Locate the reset button on the control panel
  2. Press and hold the reset button for 5 seconds
  3. Check if the system pressure gauge is between 1.0 and 1.5 bar
  4. Ensure all isolation valves under the boiler are fully open

Parts commonly replaced

  • Main PCB
  • Fan assembly
  • Air pressure switch
Full explanation for Fault Image