Johnson Starley Optima E20 Fault codes & diagnostics

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

Regular Natural Gas 5.8 kW Discontinued

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32 High 4 Medium
Just want the most common Johnson Starley Optima E20 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 failed to light or detect a flame correctly and has entered a safety lockdown.

High DIY-safe £100-£350

What to check first

  1. Check that your gas pre-payment meter (if applicable) has credit.
  2. Verify that other gas appliances in your home, like a hob, are working.
  3. Press the reset button once to see if the boiler restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe
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 £100-300

What to check first

  1. Check that your gas emergency control valve is open and you have credit on your gas meter
  2. Press the reset button on the boiler control panel once
  3. Ensure the external flue terminal is not obstructed by debris or snow

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve

Related codes

Full explanation for 13
21

Your boiler’s internal control board has detected a digital communication error while checking its own sensors.

High Engineer only £180-350

Parts commonly replaced

  • Main PCB
  • Wiring Loom
Full explanation for 21
25

Your boiler's computer has encountered a memory processing error and cannot communicate correctly with its internal software.

High DIY-safe £250-400

What to check first

  1. Turn the boiler power switch off
  2. Wait 30 seconds
  3. Turn the power back on to see if the system resets

Parts commonly replaced

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

Related codes

Full explanation for 25
26

Your boiler is starting up but failing to keep its flame lit, causing it to shut down for safety after several failed attempts.

High Engineer only £120-350

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve
  • PCB

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 £90-160

Parts commonly replaced

  • Flow temperature sensor (NTC thermistor)
  • Wiring harness connection

Related codes

Full explanation for 30
31

Your boiler has detected a fault with the sensor that monitors the temperature of the water going to your radiators, meaning it cannot heat your home safely.

High Engineer only £100-180

Parts commonly replaced

  • Central heating flow NTC sensor
  • Wiring harness / connector

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 tell how hot your water is.

Medium Engineer only £90-160

Parts commonly replaced

  • DHW Temperature Sensor (NTC thermistor)
  • Connecting wiring harness

Related codes

Full explanation for 32
34

Your boiler is not receiving enough electrical power from the main supply to operate safely and has shut down as a precaution.

High Engineer only £100-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Voltage Regulator
Full explanation for 34
37

Your boiler has detected that there is not enough water in the central heating system to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop, which is usually a flexible silver hose under or near the boiler.
  2. Open the small valves on the hose slowly until you hear water flowing and watch the pressure gauge.
  3. Close both valves tightly once the pressure gauge needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valves
  • Expansion vessel
  • Pressure sensor
Full explanation for 37
Showing 11–20 of 36
40

Your boiler has detected that there is too much water in the central heating system, causing the pressure to rise above a safe level.

Medium DIY-safe £90-180

What to check first

  1. Check that the filling loop valves are fully closed and not letting in more 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 the normal range (typically 1.0 to 1.5 bar)
  4. Reset the boiler once the pressure has been lowered

Parts commonly replaced

  • Expansion Vessel
  • Pressure Relief Valve (PRV)
  • Filling Loop Valve
Full explanation for 40
41

Your boiler has lost its connection with the water pressure sensor, which means it cannot confirm if there is enough water inside to run safely.

High Engineer only £120-£280

Parts commonly replaced

  • Water Pressure Sensor
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for 41
45

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

High Engineer only £120-£220

Parts commonly replaced

  • Flue gas temperature sensor
  • Wiring harness
  • Control board

Related codes

Full explanation for 45
46

Your boiler has shut down because a sensor that monitors the safety of exhaust gases has disconnected or failed.

High Engineer only £90-180

Parts commonly replaced

  • Flue gas temperature sensor
  • Wiring harness connector

Related codes

Full explanation for 46
47

Your boiler has lost its connection to the sensor that monitors water pressure, meaning it cannot confirm there is enough water to run safely.

High Engineer only £120-£210

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness
  • Main control board

Related codes

Full explanation for 47
E01

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

High DIY-safe £100-300

What to check first

  1. Check that your gas supply is active and that other gas appliances work.
  2. Ensure there is credit on your gas meter if you have a pre-payment meter.
  3. Press the reset button on the control panel once to see if the boiler restarts.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Spark generator

Related codes

Full explanation for E01
E02

Your boiler has detected a flame when there shouldn't be one, and has shut down as a safety precaution.

High Engineer only £120-280

Parts commonly replaced

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

Related codes

Full explanation for E02
E03

Your boiler has become too hot and has shut itself down as a safety precaution to prevent damage.

High DIY-safe £120-350

What to check first

  1. Allow the boiler to cool down for at least 30 minutes
  2. Check that all radiator valves are open and not blocked by furniture
  3. Check if the system pressure is too low and top it up if necessary
  4. Press the reset button on the control panel

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Temperature Sensor (NTC thermistor)

Related codes

Full explanation for E03
E05

Your boiler has stopped working because the fan that removes 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 after several attempts and has locked itself out for safety.

High Engineer only £120-£280

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve
  • PCB

Related codes

Full explanation for E08
Showing 21–30 of 36
E09

Your boiler has stopped working because the internal control board is receiving incorrect signals from the gas valve.

High Engineer only £180-350

Parts commonly replaced

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

Related codes

Full explanation for E09
E12

Your boiler’s internal computer chip has encountered a memory error, causing the system to lock itself down for safety.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • EEPROM Chip

Related codes

Full explanation for E12
E15

Your boiler has shut down because its internal sensors are providing conflicting temperature readings and require a safety reset or replacement.

High Engineer only £120-220

Parts commonly replaced

  • NTC thermistor
  • Flow/Return sensor
  • Control board

Related codes

Full explanation for E15
E16, E17, E18

Your boiler has detected that an internal temperature sensor is giving a fixed or faulty reading, causing the system to shut down for safety.

High Engineer only £120-£240

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Return thermistor (NTC sensor)
  • Wiring harness

Related codes

Full explanation for E16, E17, E18
E21

Your boiler's electronic control board has detected a data processing error and has shut down as a safety precaution.

High Engineer only £180-280

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 returning to the unit has developed a fault or is disconnected.

High Engineer only £120-190

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness
  • Control PCB

Related codes

Full explanation for E33
E34

Your boiler has stopped working because it isn't receiving enough stable electrical power to operate safely.

High Engineer only £120-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Internal Wiring Loom

Related codes

Full explanation for E34
E35

Your boiler's internal temperature sensor is reporting incorrect information, causing the system to stop working as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness connector

Related codes

Full explanation for E35
F07

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

High Engineer only £140-£260

Parts commonly replaced

  • Flue gas NTC sensor
  • Wiring harness
  • Main PCB
Full explanation for F07
F13

Your boiler has stopped working because it has been locked out after a problem was detected and too many attempts were made to reset it remotely.

High DIY-safe £90-250

What to check first

  1. Locate the main power switch for the boiler (usually a fused spur nearby).
  2. Turn the power off for 30 seconds.
  3. Turn the power back on and press the physical Reset button on the boiler control panel.

Parts commonly replaced

  • Primary PCB
  • User Interface Control Board
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.

High DIY-safe £0-120

What to check first

  1. Locate the external filling loop, which is usually a silver flexible hose underneath or near the boiler.
  2. Slowly open the small valves at either end of the hose until you hear water flowing.
  3. Watch the pressure gauge on the front of the boiler and close the valves once the needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

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

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

Medium DIY-safe £0-120

What to check first

  1. Check the filling loop valves are fully closed
  2. Bleed water from a radiator via the bleed valve to reduce pressure
  3. Monitor the pressure gauge to ensure it returns to between 1.0 and 1.5 bar
  4. Reset the boiler once the correct pressure is reached

Parts commonly replaced

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

Your boiler has lost its connection to the sensor that monitors water pressure, meaning it cannot tell if it is safe to operate.

High Engineer only £90-180

Parts commonly replaced

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

Your boiler's temperature sensor for hot water has stopped working properly, which means you might not get any hot water from your taps.

Medium Engineer only £100-180

Parts commonly replaced

  • DHW thermistor (NTC sensor)
  • Sensor wiring harness
Full explanation for F52
F53

Your boiler has shut down because it thinks the exhaust gases are becoming too hot, which is a safety feature to protect the internal components.

High Engineer only £120-£250

Parts commonly replaced

  • Flue Gas NTC Sensor
  • Heat Exchanger cleaning
  • Main PCB

Related codes

Full explanation for F53
Fault Image

Your boiler has detected an internal logic or performance error and has stopped working to protect itself.

High Engineer only £120-350

Parts commonly replaced

  • Electronic Control Board (PCB)
  • Air Pressure Switch
  • Thermistor
Full explanation for Fault Image