Keston Celsius 30 Fault codes & diagnostics

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

System Natural Gas 30 kW 2013-2017 Discontinued

Replaced by: Keston System 30

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Severity at a glance

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

All 50 documented codes

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C0

Your boiler's electronic brains are having trouble communicating with an internal plug-in memory chip, preventing the unit from starting up.

High Engineer only £120-300

Parts commonly replaced

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

Related codes

C2

Your boiler has lost communication with its electronic identity card, meaning the internal computer no longer knows how to operate the system safely.

High Engineer only £180-350

Parts commonly replaced

  • Boiler Chip Card (BCC)
  • Printed Circuit Board (PCB)
E00

Your boiler thinks it sees a flame even though it hasn't finished its start-up sequence, which is preventing it from running safely.

High Engineer only £120-£250

Parts commonly replaced

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

Related codes

E01

Your boiler has tried to light several times but cannot establish a steady flame, usually because there is no gas reaching the burner or the ignition sequence is blocked.

High DIY-safe £100-280

What to check first

  1. Check if other gas appliances in your home, like a hob, are working to confirm your gas supply is active.
  2. Ensure your gas meter has credit if you are on a prepaid meter.
  3. Check that the white condensate pipe leaving the boiler isn't frozen if it is cold outside.
  4. Reset the boiler by following the instructions on the display panel.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe
  • Ignition Lead

Related codes

E02

Your boiler thinks a flame is burning even when it shouldn't be, which is usually caused by an electrical fault or a sensor issue.

High Engineer only £120-£350

Parts commonly replaced

  • Ionisation Probe
  • Flame Sensing Electrode
  • 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 £90-280

What to check first

  1. Check that the boiler pressure gauge is between 1.0 and 1.5 bar.
  2. Ensure all radiator valves and internal isolation valves are fully open.
  3. Bleed your radiators to remove any trapped air from the system.

Parts commonly replaced

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

Related codes

E04

Your boiler has lost power or was switched off while it was already trying to tell you about a different fault.

High DIY-safe £80-120

What to check first

  1. Locate the reset button on the boiler control panel
  2. Press and hold the reset button for 2 to 5 seconds
  3. Wait to see if a new fault code appears, as E04 often hides the original problem

Parts commonly replaced

  • None (usually a reset issue)
  • PCB (if code persists)

Related codes

E05

Your boiler has stopped working because it cannot spin the internal fan required to safely clear exhaust gases.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Main PCB (Control board)
  • Wiring harness

Related codes

E07

Your boiler has sensed that the exhaust gases are getting too hot, which often indicates the main heat exchanger is blocked or struggling to transfer heat correctly.

High Engineer only £150-£600

Parts commonly replaced

  • Main Heat Exchanger
  • Flue NTC Sensor
  • Return NTC Sensor

Related codes

E08

Your boiler is failing to detect a flame correctly, which prevents it from firing up and providing heat or hot water.

High Engineer only £180-450

Parts commonly replaced

  • Main PCB (Control Board)
  • Ignition Electrode
  • Flame Sensing Probe

Related codes

Showing 11–20 of 50
E09

Your boiler has stopped working because its internal computer cannot properly communicate with or control the gas valve.

High Engineer only £250-£450

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

E10

Your boiler's internal computer has encountered a critical error and can no longer communicate with the rest of the system.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Control Board)
  • Wiring Harness

Related codes

E11

Your boiler has stopped working because its internal temperature sensors are giving conflicting or incorrect readings during its self-test.

High Engineer only £120-220

Parts commonly replaced

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

Your boiler’s internal computer has detected a technical malfunction that is preventing the system from running safely.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (Control Board)
  • Internal Wiring Harness

Related codes

E18

Your boiler has become too hot because water is not moving through the system quickly enough to cool it down.

High DIY-safe £150-£450

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the system water pressure is between 1.0 and 1.5 bar
  3. Bleed all radiators to remove trapped air
  4. Reset the boiler using the control panel

Parts commonly replaced

  • Circulation pump
  • Flow thermistor
  • Overheat thermostat

Related codes

E19

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

High DIY-safe £100-£350

What to check first

  1. Check that at least two or three radiators have their valves fully open.
  2. Ensure your central heating pump is running and hasn't become stuck.
  3. Check that the external bypass valve (if fitted) is not closed.

Parts commonly replaced

  • Central heating pump
  • Return temperature sensor (NTC)
  • Main heat exchanger (flushing requirement)

Related codes

E23

Your boiler is having trouble detecting or maintaining the correct water level, which has caused it to stop running for safety.

High DIY-safe £90-180

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1.0 and 1.5 bar.
  2. If the pressure is too low, use the external filling loop valves to top up the system with water.
  3. Reset the boiler by turning the control knob to 'Reset' for a few seconds.

Parts commonly replaced

  • Water pressure sensor
  • Pressure switch
  • Expansion vessel recharge

Related codes

E24

Your boiler has shut down because it keeps losing water pressure and has failed to stay topped up four times in a single day.

High DIY-safe £100-£350

What to check first

  1. Check all radiators, valves, and visible pipework for signs of a water leak.
  2. Repressurise the system to 1.5 bar using the external filling loop.
  3. Reset the boiler to clear the fault code.
  4. If the pressure drops again quickly, turn off the boiler and call a professional.

Parts commonly replaced

  • Expansion Vessel
  • Pressure Relief Valve
  • Auto Air Vent
  • Radiator Valve

Related codes

E25

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

High Engineer only £250-£450

Parts commonly replaced

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

Related codes

E26

Your boiler is struggling to keep its flame lit, which is often caused by a blocked air pipe, a frozen drain, or an issue with the gas supply.

High DIY-safe £100-280

What to check first

  1. Check the white plastic condensate pipe outside for ice and pour warm (not boiling) water over it to melt any blockages.
  2. Inspect the external flue and air terminals to ensure they aren't blocked by leaves, debris, or snow.
  3. If you use LPG, verify that your tank has sufficient fuel remaining.

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Condensate trap

Related codes

Showing 21–30 of 50
E30

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

High Engineer only £120-210

Parts commonly replaced

  • Flow Temperature Sensor (NTC)
  • Wiring Harness

Related codes

E31

Your boiler has lost communication with the internal sensor that monitors water temperature, causing the system to shut down as a safety precaution.

High Engineer only £90-180

Parts commonly replaced

  • Flow temperature sensor (NTC thermistor)
  • Wiring harness

Related codes

E32

Your boiler has detected a fault with the sensor that monitors your hot water temperature, meaning you likely have no hot water or the temperature is inconsistent.

High Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC Sensor)
  • Wiring Harness

Related codes

E33

Your boiler has detected an electrical fault with the sensor that manages your hot water, which usually means your taps will only run cold.

High Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC sensor)
  • Wiring harness connector

Related codes

E34

Your boiler has detected that the electrical power coming into your home is too low for it to operate safely.

High Engineer only £100-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External Voltage Stabiliser

Related codes

E35

Your boiler has detected an issue with the electrical power timing coming from the mains, which is preventing it from running safely.

High Engineer only £100-350

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Power Supply Unit

Related codes

E36

Your boiler has lost its connection to the sensor that monitors the temperature of the water leaving the unit, causing it to shut down for safety.

High Engineer only £90-160

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness connector

Related codes

E37

Your boiler has detected that the water pressure is too low, causing it to stop working for safety.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver braided hose with small valves) underneath or near the boiler.
  2. Slowly open the valve or valves until you hear water entering the system.
  3. Watch the pressure gauge and close the valves once the needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Pressure sensor
  • Expansion vessel

Related codes

E38

Your boiler's control panel button is stuck or faulty, preventing the system from processing commands or resetting.

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 has caused it to shut down for safety.

High DIY-safe £80-180

What to check first

  1. Check that the silver filling loop valves are tightly closed.
  2. Bleed water from a radiator using a radiator key to lower the pressure.
  3. Monitor the pressure gauge until it returns to between 1.0 and 1.5 bar.
  4. Press the reset button once the pressure is stabilised.

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure relief valve

Related codes

Showing 31–40 of 50
E41

Your boiler’s internal control panels are having trouble talking to each other, meaning the main computer cannot receive instructions to heat your home.

High Engineer only £120-350

Parts commonly replaced

  • Wiring harness
  • Display fascia panel
  • Main PCB (Printed Circuit Board)

Related codes

E43

Your boiler has detected a technical fault with the sensor that monitors water temperature returning to the unit, causing the system to shut down for safety.

High Engineer only £90-180

Parts commonly replaced

  • Return NTC thermistor
  • Sensor wiring harness

Related codes

E44

Your boiler has stopped working because it cannot communicate with the sensor that monitors the temperature of the water returning to the unit.

High Engineer only £90-160

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness / Connector

Related codes

E45

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 £120-220

Parts commonly replaced

  • Flue Gas NTC Sensor
  • Wiring Harness

Related codes

E46

Your boiler's internal computer brain is having trouble talking to the display panel on the front, which stops the system from working correctly.

High Engineer only £120-350

Parts commonly replaced

  • Wiring harness
  • Display PCB
  • Main Control Board

Related codes

E99

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 £120-£220

Parts commonly replaced

  • Flue Gas Thermistor
  • Wiring Harness
  • Main Control Board

Related codes

Enn

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

High Engineer only £150-450

Parts commonly replaced

  • Main PCB
  • Internal Wiring Harness
  • External Sensors

Related codes

F0

Your boiler has lost its identity because a small internal memory chip is either missing or has failed, meaning the unit doesn't know how to operate.

High Engineer only £120-220

Parts commonly replaced

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

Related codes

F1

Your boiler has detected that there is not enough water in the system to operate safely and has shut down to prevent damage.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver braided hose with small valves) underneath or near the boiler.
  2. Slowly open the valves until you hear water entering the system and watch the pressure gauge.
  3. Close both valves tightly once the pressure gauge reaches 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

F2

Your boiler has lit correctly but the flame has gone out unexpectedly, preventing the system from heating your home.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances in your home, such as a hob, are working to confirm gas supply.
  2. If it is freezing outside, check that the white plastic condensate pipe leading outdoors is not frozen and blocked.
  3. Reset the boiler by pressing the 'Reset' button.

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 is not running at the correct speed or has failed to start, which means the system has safely shut down to prevent gas from entering without ventilation.

High Engineer only £250-450

Parts commonly replaced

  • Fan Assembly
  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

F4

Your boiler has detected that water is not circulating correctly or a temperature sensor has failed, causing the system to shut down for safety.

High Engineer only £120-250

Parts commonly replaced

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

Related codes

F5

Your boiler has detected a problem with the sensor that monitors water returning from your radiators, causing the system to shut down for safety.

High Engineer only £120-£200

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness
  • Main control board

Related codes

F6

Your boiler is having trouble communicating with the outdoor thermometer that helps it adjust to the weather, which may affect its efficiency.

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 coming into your home is at a lower voltage than it needs to operate safely.

Medium Engineer only £100-350

Parts commonly replaced

  • Main Control PCB
  • Electrical supply wiring

Related codes

F9

Your boiler's main control computer has developed an internal error and can no longer communicate with the rest of the system.

High Engineer only £250-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • PCB Wiring Harness

Related codes

L1

Your boiler has shut down because the water inside has become too hot, which is usually caused by a circulation problem or a blockage.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open to allow water to flow.
  2. Verify that the boiler pressure gauge is between 1 and 1.5 bar, and top up if it is low.
  3. Press the reset button on the front of the boiler once the unit has cooled down.

Parts commonly replaced

  • Circulation pump
  • Flow thermistor (NTC sensor)
  • Main heat exchanger (flushing/cleaning)

Related codes

L2

Your boiler has started an ignition sequence but lost the flame, meaning the system has shut down for safety because it cannot stay lit.

High DIY-safe £100-350

What to check first

  1. Check that your gas prepayment meter has credit (if applicable)
  2. Ensure your external gas stop tap is fully open
  3. Press the 'Reset' button on the front of the boiler for two seconds

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve
  • Condensate trap (if blocked)

Related codes

L5

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

High DIY-safe £0-120

What to check first

  1. Turn off the power supply to the boiler at the fused spur switch
  2. Wait for at least 5 minutes to allow the internal timer to clear
  3. Turn the power back on and wait for the boiler to restart naturally

Parts commonly replaced

  • No parts usually required for L5 itself
  • Ignition Electrode
  • Gas Valve

Related codes

L6

Your boiler has sensed a flame inside the unit when there shouldn't be one, causing it to shut down for safety purposes.

High Engineer only £120-350

Parts commonly replaced

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

Related codes