Keston 30C Fault codes & diagnostics

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

Combi Natural Gas 30 kW 2010 onwards Discontinued
GC Number 47-930-03
Download the Keston 30C manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
26 High 2 Medium 2 Low
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-930-03 Keston 30C 2010–present
Just want the most common Keston 30C faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 30 documented codes

Tap any card for details

81

Your boiler has switched itself off because it has reached an unsafe temperature, likely due to low water pressure or trapped air in your heating system.

High DIY-safe £0-180

What to check first

  1. Check the pressure gauge and top up the system to 1.0 bar using the filling loop
  2. Bleed your radiators to remove any trapped air
  3. Ensure your radiator valves (TRVs) are turned on and open
  4. Press the reset button on the boiler control panel

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Pressure Sensor

Related codes

Full explanation for 81
82

Your boiler has stopped working because it failed to light properly or detected that the exhaust gases are getting too hot.

High DIY-safe £120-350

What to check first

  1. Check if other gas appliances in your home, such as a hob, are still working to confirm gas supply.
  2. Ensure your gas meter has credit if you have a pre-payment meter.
  3. Press the 'Reset' button on the boiler control panel once to see if the system restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flue Thermistor
  • PCB

Related codes

Full explanation for 82
83

Your boiler has detected low water pressure and has shut down as a safety precaution to avoid overheating.

High DIY-safe £60-120

What to check first

  1. Check the pressure gauge on the boiler and use the filling loop to top it up to 1.0 bar.
  2. Bleed your radiators to remove any trapped air that could be affecting water flow.
  3. Ensure all radiator valves and boiler isolation valves are fully open.
  4. Press the reset button once the pressure is restored.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Circulation pump

Related codes

Full explanation for 83
84

Your boiler detects a flame when there shouldn't be one, so it has locked itself for safety to prevent a gas-related issue.

High Engineer only £120-350

Parts commonly replaced

  • Ionisation Probe
  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 84
85

Your boiler has detected that the water pressure in the central heating system is too low to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the external filling loop (usually a silver braided hose under the boiler)
  2. Slowly open the valves until the pressure gauge on the boiler front reaches 1.0 - 1.5 bar
  3. Close the valves tightly and check if the '85' code has cleared

Parts commonly replaced

  • Filling loop
  • Expansion vessel (repressurising or replacement)
  • Pressure relief valve

Related codes

Full explanation for 85
86

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

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure your gas supply is active
  2. Check if your gas meter has credit (if using a prepaid meter)
  3. Reset the boiler by pressing the 'Reset' button

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve

Related codes

Full explanation for 86
87

Your boiler has detected an issue with the air supply system and has safely shut itself down to prevent damage.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 87
88

Your boiler has detected a problem with the temperature sensor that monitors the water leaving the unit, which means it cannot safely regulate heat.

High Engineer only £90-160

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Connecting lead or wiring harness

Related codes

Full explanation for 88
89

Your boiler has lost its ability to accurately measure the temperature of the water returning to the unit, causing it to shut down for safety.

High Engineer only £100-£180

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for 89
90

Your boiler has lost connection with the sensor that measures the temperature outside, which may cause your heating to be less efficient or unresponsive.

Low Engineer only £90-160

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring

Related codes

Full explanation for 90
Showing 11–20 of 30
91

Your boiler has detected a problem with the internal sensor that monitors the temperature of water flowing through the system, preventing it from heating up.

High Engineer only £100-£180

Parts commonly replaced

  • Flow Thermistor
  • Wiring Harness

Related codes

Full explanation for 91
92

Your boiler has developed a communication error with its internal memory chip, meaning it cannot process the settings required to run safely.

High Engineer only £180-450

Parts commonly replaced

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

Related codes

Full explanation for 92
93

Your boiler is providing hot water for your taps but is failing to heat your radiators.

Medium DIY-safe £120-280

What to check first

  1. Check that your room thermostat or smart hub is turned up and calling for heat
  2. Ensure the central heating timer or programmer is set to 'On'
  3. Replace batteries in your wireless room thermostat if applicable

Parts commonly replaced

  • Diverter Valve Actuator Motor
  • Diverter Valve Internal Cartridge
  • Room Thermostat

Related codes

Full explanation for 93
94

Your boiler is successfully heating your radiators, but a component failure is preventing it from providing hot water to your taps.

Medium Engineer only £180-350

Parts commonly replaced

  • Diverter Valve
  • Diverter Valve Motor/Actuator
  • Domestic Hot Water Sensor

Related codes

Full explanation for 94
95

Your boiler has lost power or the screen has failed, meaning the unit cannot operate or communicate its status.

High DIY-safe £120-£450

What to check first

  1. Check if your main consumer unit (fuse box) has tripped.
  2. Ensure the boiler's isolation switch (usually a fused spur near the boiler) is turned on.
  3. Check if other appliances in the house have power.

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Display Interface Panel
  • Internal Fuse

Related codes

Full explanation for 95
BCC Fault

Your boiler’s internal computer card has stopped working properly, preventing the system from operating.

High Engineer only £100-£250

Parts commonly replaced

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

Related codes

Full explanation for BCC Fault
False Flame Lockout

Your boiler thinks it sees a flame even though it isn't fire-up time, so it has locked itself for safety.

High Engineer only £120-350

Parts commonly replaced

  • Flame Sensing Electrode
  • Ignition Lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for False Flame Lockout
Fan Fault

Your boiler has detected a problem with its internal fan, which means it cannot safely blow exhaust gases out of your home and will not start.

High Engineer only £280-450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for Fan Fault
Flame Loss

Your boiler has stopped working because it cannot keep the burner lit, which is often caused by a fuel supply issue or a dirty internal sensor.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances, like a cooker, are working to confirm your gas supply is active
  2. If you have a prepay meter, check that you have sufficient credit
  3. Ensure your external condensate pipe hasn't frozen if the weather is extremely cold
  4. Reset the boiler using the 'Reset' button

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead
  • Condensate Trap

Related codes

Full explanation for Flame Loss
Flow Temperature Overheat Lockout

Your boiler has switched itself off to prevent damage because the internal water temperature has become too hot.

High DIY-safe £80-250

What to check first

  1. Check the pressure gauge and top up to 1.0 bar if it is low
  2. Ensure all radiator valves are fully open
  3. Bleed any air out of the radiators and reset the boiler

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Flow Temperature Sensor

Related codes

Full explanation for Flow Temperature Overheat Lockout
Showing 21–30 of 30
Ignition Lockout

Your boiler has failed to light after several attempts and has stopped for safety, meaning you currently have no heating or hot water.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances, like a hob, are working to confirm gas supply
  2. Check that your gas meter has credit if you use a prepay meter
  3. Press the Reset button once to see if the boiler restarts

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Probe
  • Condensate Trap (if blocked)

Related codes

Full explanation for Ignition Lockout
Low mains voltage

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

High Engineer only £0-150

Parts commonly replaced

  • None - External Supply Issue
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Low mains voltage
Low water pressure

Your boiler has detected that there is not enough water in the system to operate safely, which is usually caused by a small leak or air being bled from radiators.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.
  2. Slowly open the one or two taps/valves until you hear water flowing.
  3. Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps tightly.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for Low water pressure
No water flow lockout

Your boiler has stopped working because it cannot detect water moving through the system to keep it cool and functioning safely.

High DIY-safe £100-350

What to check first

  1. Check that your boiler pressure gauge is between 1.0 and 1.5 bar.
  2. Ensure all radiator valves and isolation valves under the boiler are fully open.
  3. Gently bleed your radiators to remove any trapped air pockets.
  4. Restart the boiler using the reset button once pressure is confirmed.

Parts commonly replaced

  • Circulation Pump
  • Water Pressure Sensor
  • Flow/Return Thermistor

Related codes

Full explanation for No water flow lockout
Outside sensor fault

Your boiler has lost communication with the sensor that monitors the outdoor temperature, which may cause your heating to run less efficiently.

Low Engineer only £90-160

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Sensor wiring

Related codes

Full explanation for Outside sensor fault
Overheat lockout

Your boiler has shut itself down because the water inside has become too hot, which is a safety measure to prevent damage.

High DIY-safe £120-350

What to check first

  1. Check that your radiator valves/TRVs are open to allow water to flow.
  2. Check the boiler pressure gauge and top up to 1.5 bar if it is low.
  3. Banish any trapped air by bleeding your radiators.
  4. Press the Reset button once the boiler has had 15 minutes to cool down.

Parts commonly replaced

  • Circulating Pump
  • Overheat Thermostat
  • Main Heat Exchanger (Flushing)
  • Thermistor

Related codes

Full explanation for Overheat lockout
PCB Fault

Your boiler's internal computer brain has malfunctioned or lost its programming, meaning the unit cannot communicate with its components to provide heating or hot water.

High Engineer only £350-£550

Parts commonly replaced

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

Related codes

Full explanation for PCB Fault
Return Thermistor Fault

Your boiler has stopped working because a sensor that monitors the temperature of the water returning to the unit has failed or disconnected.

High Engineer only £120-220

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring Harness

Related codes

Full explanation for Return Thermistor Fault
Too many resets

Your boiler has been manually reset too many times in a short window and has temporarily locked itself to prevent potential damage or unsafe operation.

High DIY-safe £100-350

What to check first

  1. Turn off the power supply to the boiler at the fused spur switch (the wall switch).
  2. Wait approximately 30 seconds.
  3. Turn the power back on to clear the lockout memory and see if the boiler resumes normal operation.

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Electrode
  • Gas Valve
  • PCB

Related codes

Full explanation for Too many resets
Status & diagnostic codes

1 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

Reset the boiler Diagnostic Your boiler has likely had a major internal part replaced recently and simply needs a manual restart to confirm its settings and get back to work.