Johnson Starley Warmaheat Fault codes & diagnostics

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

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33 High 3 Medium

All 36 documented codes

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12

Your boiler has stopped working as a safety precaution due to an internal system failure that prevents it from starting correctly.

High Engineer only £100-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Ignition Electrode
  • Gas Valve
13

Your boiler has tried to light several times but has failed to stay on, so it has locked itself out for safety.

High DIY-safe £100-£250

What to check first

  1. Check that your gas pre-pay meter has credit (if applicable)
  2. Check that other gas appliances like your hob are working to ensure gas supply
  3. Press the reset button on the boiler control panel once

Parts commonly replaced

  • Ignition electrode
  • Flame sensing probe
  • Gas valve

Related codes

21

Your boiler’s internal control board is having trouble reading its own data correctly, which prevents the system from starting safely.

High Engineer only £180-£350

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Harness

Related codes

25

Your boiler's computer memory has encountered a processing error, which is preventing the system from starting up safely.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (Control Board)
  • User Interface

Related codes

26

Your boiler is repeatedly struggling to keep the flame lit and has locked itself out for safety.

High Engineer only £120-350

Parts commonly replaced

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

Related codes

30

Your boiler has stopped working because its internal thermometer has developed an electrical fault and can no longer monitor the water temperature accurately.

High Engineer only £90-160

Parts commonly replaced

  • Flow temperature sensor (NTC thermistor)
  • Connecting wiring harness

Related codes

31

Your boiler's internal thermometer that monitors the temperature for your radiators has developed a fault and can no longer read the temperature correctly.

High Engineer only £90-180

Parts commonly replaced

  • Central heating flow NTC thermistor
  • Wiring harness
  • Main PCB

Related codes

32

Your boiler has detected a failure with the sensor that monitors your hot water temperature, meaning it cannot safely heat water for your taps.

High Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC Sensor)
  • Wiring harness

Related codes

34

Your boiler has stopped working because the electrical power coming from your home's mains supply is too low for it to operate safely.

High Engineer only £90-250

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Voltage Stabiliser
37

Your boiler has detected that the water pressure in the central heating system is too low and it has stopped working as a safety precaution.

High DIY-safe £0-160

What to check first

  1. Locate the external filling loop, which is usually a silver braided flexible hose under or near the boiler.
  2. Slowly open the one or two valves on the hose to let water into the system.
  3. Watch the pressure gauge and close the valves tightly once the needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel re-pressurisation
  • Pressure sensor
Showing 11–20 of 36
40

Your boiler has detected that the water pressure inside the system is too high, which could cause stress on the internal pipes and components.

Medium DIY-safe £80-200

What to check first

  1. Check that the filling loop valves are fully closed
  2. Bleed water from a radiator using a bleed key to reduce pressure
  3. Monitor the pressure gauge to ensure it returns to between 1 and 1.5 bar
  4. If pressure continues to rise, contact a Gas Safe engineer

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure relief valve

Related codes

41

Your boiler has stopped working because it cannot detect any water pressure, which means it cannot safely heat your home.

High DIY-safe £80-180

What to check first

  1. Locate the pressure gauge on the front of the boiler or underneath it.
  2. If the needle is below 1 bar, find the filling loop (a flexible silver hose).
  3. Slowly open the valve(s) until the pressure reaches 1.5 bar, then close them tightly.
  4. Press the 'Reset' button on the control panel.

Parts commonly replaced

  • Water pressure switch
  • Expansion vessel
  • Man-Machine Interface (MMI) PCB

Related codes

45

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

High Engineer only £140-£260

Parts commonly replaced

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

Related codes

46

Your boiler has shut down because it can no longer safely monitor the temperature of the exhaust gases leaving the system.

High Engineer only £120-210

Parts commonly replaced

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

Related codes

47

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

High Engineer only £120-210

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness
  • Main PCB

Related codes

E01

Your boiler has tried to light several times but failed, so it has locked itself for safety.

High DIY-safe £100-280

What to check first

  1. Check that your gas prepayment meter has credit
  2. Ensure your external gas stopcock is fully open
  3. Press and hold the reset button for five seconds

Parts commonly replaced

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

Related codes

E02

Your boiler thinks it sees a flame even though it hasn't actually started the ignition process, causing it to shut down for safety.

High Engineer only £120-£280

Parts commonly replaced

  • Flame rectification probe
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes

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. Check that all radiator valves are open to allow water to flow.
  2. Ensure there is enough water pressure in the system by checking the pressure gauge.
  3. Wait for the boiler to cool down and try pressing the reset button.

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Main heat exchanger

Related codes

E05

Your boiler has detected an issue with the fan that safely removes exhaust gases, meaning the system has shut down for your safety.

High Engineer only £250-450

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • Main PCB

Related codes

E08

Your boiler is failing to detect the flame it has lit, causing it to shut down as a safety precaution.

High Engineer only £120-300

Parts commonly replaced

  • Flame rectification probe
  • Ignition electrode
  • PCB

Related codes

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
  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

E12

Your boiler's computer memory has encountered a deep internal error and can no longer process instructions safely.

High Engineer only £280-550

Parts commonly replaced

  • Main Printed Circuit Board (PCB)

Related codes

E15

Your boiler has shut down because a temperature sensor is giving inconsistent readings and no longer trusts the data it is receiving.

High Engineer only £120-£220

Parts commonly replaced

  • Combined thermistor (NTC sensor)
  • Wiring harness
  • Main PCB

Related codes

E16, E17, E18

Your boiler has stopped working because its internal temperature sensors are giving conflicting readings or appear to be stuck, causing the system to shut down for safety.

High Engineer only £120-240

Parts commonly replaced

  • NTC Temperature Sensor
  • Flow/Return Thermistor
  • Main Control Board

Related codes

E21

Your boiler has shut itself down because it has detected a technical error within its electronic control system.

High Engineer only £180-350

Parts commonly replaced

  • Main PCB
  • Control Board

Related codes

E33

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning to the unit, which is preventing it from heating properly.

High Engineer only £110-190

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Connecting wiring harness

Related codes

E34

Your boiler has shut down because the electrical power reaching it is too weak or unstable to operate safely.

High Engineer only £120-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Internal Wiring Harness
  • External Voltage Stabilizer

Related codes

E35

Your boiler has stopped working because it cannot correctly measure the temperature of the water leaving the unit.

High Engineer only £100-£190

Parts commonly replaced

  • Flow Thermistor (NTC sensor)
  • Connecting wiring harness

Related codes

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 £120-240

Parts commonly replaced

  • Exhaust thermistor
  • Flue sensor wiring loom
  • Main PCB
F13

Your boiler has shut itself down for safety after too many failed attempts to reset it remotely or due to a persistent internal fault.

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 five seconds.
  3. Wait for the boiler to complete its startup sequence; if the code returns, contact a professional.

Parts commonly replaced

  • Main PCB
  • Ignition Electrode
  • Gas Valve
Showing 31–36 of 36
F37

Your boiler has detected that the water pressure in the system is too low to operate safely and has shut down to protect itself.

High DIY-safe £0-120

What to check first

  1. Locate the metal braided filling loop underneath or near the boiler
  2. Open the valves slowly until you hear water flowing into the system
  3. Watch the pressure gauge until it reaches between 1 and 1.5 bar
  4. Close both valves completely and restart the boiler if necessary

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel
F40

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

Medium DIY-safe £80-160

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 lower the system pressure
  3. Monitor the pressure gauge until it returns to the normal range (usually 1.0 to 1.5 bar)
  4. Reset the boiler once the pressure is stabilised

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure relief valve (PRV)
F47

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

High Engineer only £120-220

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness
  • Main PCB
F52

Your boiler is having trouble measuring the temperature of your tap water, which usually means your hot water won't heat up correctly.

Medium Engineer only £90-160

Parts commonly replaced

  • DHW thermistor (NTC sensor)
  • Wiring harness connection
F53

Your boiler has shut down because a sensor has detected that the exhaust gases are getting too hot, which is a safety measure to prevent damage.

High Engineer only £120-£250

Parts commonly replaced

  • Flue temperature sensor (NTC)
  • Main heat exchanger (cleaning/flush)
  • Printed Circuit Board

Related codes

Fault Image

Your boiler has detected an internal control error and has locked itself out to ensure your safety.

High Engineer only £150-450

Parts commonly replaced

  • Main PCB
  • Ignition Electrode
  • Flame Sensor