Keston Heat 55 Fault codes & diagnostics

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

System Natural Gas 55 kW 2013-2022 Discontinued

Replaced by: Keston Heat 55 2

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Each cell = one fault code. Hover to identify.
47 High 2 Medium 1 Low

All 50 documented codes

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C0

Your boiler has failed to recognize its internal identity chip, preventing it from starting up correctly.

High Engineer only £150-£450

Parts commonly replaced

  • Boiler Chip Card (BCC)
  • Printed Circuit Board (PCB)

Related codes

C2

Your boiler's identity card has stopped communicating with the main control board, preventing the system from operating.

High Engineer only £180-350

Parts commonly replaced

  • Boiler Chip Card (BCC)
  • Printed Circuit Board (PCB)

Related codes

E00

Your boiler thinks it sees a flame even though it hasn't tried to light yet, which is causing it to shut down for safety.

High Engineer only £120-£280

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Main control PCB

Related codes

E01

Your boiler has tried to light several times but cannot start, which means you currently have no heating or hot water.

High DIY-safe £100-£350

What to check first

  1. Check that your gas emergency control valve is in the 'on' position
  2. If you have a pre-payment meter, ensure it has sufficient credit
  3. Check if other gas appliances in your home, such as a hob, are working correctly
  4. Gently press the 'Reset' button on the boiler control panel

Parts commonly replaced

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

Related codes

E02

Your boiler thinks a flame is burning even when it shouldn't be, which is preventing it from starting up safely.

High Engineer only £120-350

Parts commonly replaced

  • Flame sensing electrode
  • 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 internal damage.

High DIY-safe £100-350

What to check first

  1. Check that the boiler pressure gauge is between 1.0 and 1.5 bar and top up if necessary.
  2. Ensure all radiator valves and any isolation valves under the boiler are fully open.
  3. Bleed your radiators to remove any trapped air that might be blocking water flow.

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Thermistor (NTC sensor)

Related codes

E04

Your boiler has lost power or been turned off while it was already trying to report a different fault, causing it to freeze in its lockout state.

High DIY-safe £0-120

What to check first

  1. Locate the reset button on the control panel
  2. Press and hold the reset button for five seconds
  3. Wait for the boiler to restart and check for a new fault code that may have caused the original lockout

Parts commonly replaced

  • None (Software reset usually sufficient)
  • Printed Circuit Board (PCB) if fault persists

Related codes

E05

Your boiler has stopped working because the internal fan, which safely pushes exhaust gases out of the flue, is not spinning correctly.

High Engineer only £250-450

Parts commonly replaced

  • Combustion Fan
  • Fan Wiring Harness
  • Main PCB

Related codes

E07

Your boiler has shut down because the internal exhaust gases are getting too hot, indicating that heat is not being transferred correctly into your home's water system.

High Engineer only £180-650

Parts commonly replaced

  • Main Heat Exchanger
  • Flue Thermistor
  • Return Water Sensor

Related codes

E08

Your boiler is having trouble detecting the flame used to heat your water, which is causing the system to shut down for safety.

High Engineer only £250-£550

Parts commonly replaced

  • Main Control Board
  • Flame Sensing Electrode
  • Wiring Harness

Related codes

Showing 11–20 of 50
E09

Your boiler has detected an internal electrical problem with the parts that control the gas flow, meaning it cannot ignite for safety reasons.

High Engineer only £250-£550

Parts commonly replaced

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

Related codes

E10

Your boiler's internal computer has encountered a critical electronic failure and cannot safely manage the heating process.

High Engineer only £350-£550

Parts commonly replaced

  • Main PCB (Control Board)
  • Wiring Harness

Related codes

E11

Your boiler has stopped working because its internal sensors are reporting conflicting temperature readings from your pipework.

High Engineer only £120-£250

Parts commonly replaced

  • Flow temperature NTC sensor
  • Return temperature NTC sensor
  • Wiring harness
E12 to E22

Your boiler's main internal computer has encountered a critical error and is unable to manage the heating process safely.

High Engineer only £350-£550

Parts commonly replaced

  • Main PCB (Control Board)
  • Display PCB

Related codes

E18

Your boiler has detected that the water inside is getting too hot because it is not moving around your heating system quickly enough.

High DIY-safe £150-£450

What to check first

  1. Check that at least two or three radiator valves are fully open to allow water flow.
  2. Ensure your external heating pump (if visible) is running and not making a loud humming noise.
  3. Check your boiler pressure gauge and top it up to 1.5 bar if it is too low.

Parts commonly replaced

  • External Circulation Pump
  • Heat Exchanger (Powerflush required)
  • Flow/Return Thermistors
E19

Your boiler has shut down because the water returning to it is too hot, which usually means the water isn't moving through your radiators properly.

High DIY-safe £120-£350

What to check first

  1. Check that at least two or three of your radiator valves are turned to the highest setting
  2. Ensure all radiator valves are fully open and not stuck shut
  3. Check if your central heating pump is making an unusual noise or vibrating

Parts commonly replaced

  • Central heating pump
  • Return water temperature sensor (NTC thermistor)
  • External system filter (if blocked)

Related codes

E23

Your boiler is having trouble detecting or maintaining the correct water level needed to operate safely.

High Engineer only £120-£250

Parts commonly replaced

  • Water pressure sensor
  • Pressure transducer
  • Wiring harness

Related codes

E24

Your boiler has shut down because it keeps losing water pressure and needs a top-up or has a leak that needs investigation.

High DIY-safe £100-350

What to check first

  1. Check the pressure gauge; if it is below 1 bar, use the external filling loop to top up the system to 1.5 bar.
  2. Inspect all visible radiators, valves, and pipework for any signs of water leaks.
  3. Reset the boiler to clear the code and monitor the pressure gauge for further drops.

Parts commonly replaced

  • Expansion Vessel
  • Pressure Relief Valve
  • Automatic Air Vent

Related codes

E25

Your boiler's internal computer has encountered a critical error and can no longer safely manage the heating process.

High Engineer only £350-£550

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Control Box Unit

Related codes

E26

Your boiler has started an ignition but the flame has unexpectedly gone out, causing the system to shut down for safety.

High DIY-safe £100-£350

What to check first

  1. Check the outdoor condensate drain pipe for any blockages or ice if it is freezing outside.
  2. Inspect the outdoor flue terminal to ensure it is not obstructed by leaves, nests, or debris.
  3. Ensure your gas supply is active and, if using LPG, check that the tank is not empty.
  4. Reset the boiler once you have checked for external obstructions.

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead

Related codes

Showing 21–30 of 50
E30

Your boiler has stopped working because its internal computer has lost communication with the sensor that monitors water temperature leaving the unit.

High Engineer only £120-220

Parts commonly replaced

  • Flow Thermistor
  • Wiring Harness
  • Main Heat Interface Board

Related codes

E31

Your boiler has lost communication with the internal sensor that monitors the temperature of the water leaving the unit, causing it to stop running for safety.

High Engineer only £120-£220

Parts commonly replaced

  • Flow temperature thermistor (NTC)
  • Wiring harness / Cable connector

Related codes

E32

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

Medium Engineer only £110-180

Parts commonly replaced

  • Domestic Hot Water (DHW) thermistor
  • DHW sensor wiring harness

Related codes

E33

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

High Engineer only £90-160

Parts commonly replaced

  • Domestic hot water (DHW) thermistor sensor
  • Sensor wiring harness

Related codes

E34

Your boiler has detected that the electricity supply coming into your home is too weak for it to operate safely.

High Engineer only £90-250

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External Power Transformer

Related codes

E35

Your boiler is struggling to operate because it detects an inconsistency in the electricity supply entering your home.

High Engineer only £100-350

Parts commonly replaced

  • Main PCB
  • External Voltage Regulator

Related codes

E36

Your boiler has stopped working because it can no longer accurately measure the temperature of the water leaving the unit.

High Engineer only £90-160

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness

Related codes

E37

Your boiler has stopped working because the water pressure inside the system is too low.

High DIY-safe £0-120

What to check first

  1. Locate the external filling loop, which is usually a flexible silver hose connected to the pipework.
  2. Slowly open both taps on the filling loop to allow water into the system.
  3. Wait for the pressure gauge on the boiler to reach between 1 and 1.5 bar, then close both taps tightly.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

E38

Your boiler's control panel has developed an internal electrical fault and is no longer responding to the reset button.

High Engineer only £180-£320

Parts commonly replaced

  • Fascia PCB
  • User Interface Panel

Related codes

E40

Your boiler has detected that there is too much water pressure in the system, which can cause internal leaks or damage if not reduced.

Medium DIY-safe £80-200

What to check first

  1. Locate a radiator bleed valve and use a key to release water into a cloth or container
  2. Monitor the pressure gauge on the boiler until it drops between 1.0 and 1.5 bar
  3. Ensure the filling loop valves are fully closed and not letting in more water
  4. Press the reset button once the pressure has been lowered

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure Relief Valve (PRV)

Related codes

Showing 31–40 of 50
E41

Your boiler's main control panel has lost contact with the internal computer that runs the system, preventing it from starting up.

High Engineer only £120-450

Parts commonly replaced

  • Fascia PCB
  • Main Control PCB
  • Communication Ribbon Cable

Related codes

E43

Your boiler has stopped working because it cannot correctly read the temperature of the water coming back into the system.

High Engineer only £120-£200

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness connector

Related codes

E44

Your boiler has lost communication with the sensor that monitors the temperature of the water coming back from your radiators, causing the system to stop working for safety.

High Engineer only £100-190

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness connector

Related codes

E45

Your boiler has stopped working because it cannot correctly read the temperature of the exhaust gases, usually due to a faulty electrical connection or sensor.

High Engineer only £120-£210

Parts commonly replaced

  • Flue Gas Sensor
  • Wiring harness
  • Main PCB

Related codes

E46

Your boiler's main control panel is having trouble talking to the internal computer, meaning the boiler cannot receive commands to turn on.

High Engineer only £120-350

Parts commonly replaced

  • Display Fascia PCB
  • Main Control PCB
  • Communication Wiring Harness

Related codes

E99

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

High Engineer only £120-£200

Parts commonly replaced

  • Flue Gas Sensor
  • Wiring Harness
  • Main Control Core (PCB)

Related codes

Enn

Your boiler has encountered a critical system error that has forced it to shut down to prevent damage or safety issues.

High Engineer only £150-£450

Parts commonly replaced

  • Main Control PCB
  • Ignition Electrode
  • Wiring Harness

Related codes

F0

Your boiler's computer memory card is missing or not making a proper connection, which means the system doesn't know how to operate.

High Engineer only £100-250

Parts commonly replaced

  • Boiler Chip Card (BCC)
  • Printed Circuit Board (PCB)

Related codes

F1

Your boiler has detected that the water pressure inside the system has dropped too low to operate safely.

High DIY-safe £0-160

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 fresh water into the system while watching the pressure gauge.
  3. Close both valves tightly once the pressure gauge reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

F2

Your boiler has started to light but the flame has gone out, meaning the system has shut down as a safety precaution.

High DIY-safe £120-£350

What to check first

  1. Check that your gas emergency control valve is open and you have credit on your gas meter
  2. If it is freezing outside, check if the white plastic condensate pipe leading outdoors is frozen and thaw it with warm water
  3. Press the 'Reset' button on the boiler control panel for two seconds

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead
  • Spark Generator

Related codes

Showing 41–50 of 50
F3

Your boiler's internal fan, which safely pushes exhaust gases out of the flue, is not spinning correctly and the system has shut down for safety.

High Engineer only £280-450

Parts commonly replaced

  • Fan motor
  • PCB Control Board
  • Wiring Harness

Related codes

F4

Your boiler has detected that water is not circulating correctly or the sensors that measure temperature are failing, causing the system to shut down for safety.

High Engineer only £120-£280

Parts commonly replaced

  • Flow thermistor
  • Return thermistor
  • Circulation pump
  • Wiring harness

Related codes

F5

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning from your radiators, preventing it from heating correctly.

High Engineer only £120-£220

Parts commonly replaced

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

Related codes

F6

Your boiler has lost connection with the sensor that measures the temperature outside, which may cause your heating to work less efficiently or run at the wrong temperature.

Low Engineer only £90-180

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring harness

Related codes

F7

Your boiler has detected that the electricity supply from the mains is too low for it to operate safely and correctly.

High Engineer only £100-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External voltage regulator

Related codes

F9

Your boiler's main internal computer has developed a fault and is unable to control the heating system correctly.

High Engineer only £350-£550

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

L1

Your boiler has shut down because the internal water temperature has become too hot.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open to allow water to circulate.
  2. Ensure the external system pump is running if visible.
  3. Try resetting the boiler by pressing the 'Reset' button.

Parts commonly replaced

  • External Pump
  • Heat Exchanger (Flushing)
  • Flow Temperature Sensor
  • System Filter

Related codes

L2

Your boiler has lost its flame during operation, meaning it can no longer safely heat your water or radiators.

High DIY-safe £120-350

What to check first

  1. Check if your gas credit has run out or if other gas appliances in your home are working
  2. Check that the condensate pipe (the white plastic pipe outside) hasn't frozen in cold weather
  3. Press the 'Reset' button on the boiler control panel

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead
  • Spark Generator

Related codes

L5

Your boiler has been manually reset too many times in a short period and has locked itself out as a safety precaution.

High DIY-safe £0-120

What to check first

  1. Switch the electricity supply to the boiler off at the fused spur switch
  2. Wait for 30 seconds
  3. Switch the power back on to clear the temporary lockout

Parts commonly replaced

  • No parts usually required
  • Ignition Electrode (if underlying ignition fault exists)
  • PCB (if software fault)

Related codes

L6

Your boiler thinks there is a flame present when there shouldn't be one, so it has shut down for safety purposes.

High Engineer only £100-280

Parts commonly replaced

  • Flame sense electrode
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes