Keston Combi C30 Fault codes & diagnostics

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

Combi Natural Gas 30 kW 2016 onwards
GC Number 47-930-07
Download the Keston Combi C30 manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 22 High 1 Medium 1 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
47-930-07 COMBI C30 2016–present

Appliance names and production years are taken from the UK Product Characteristics Database (the SAP/SEDBUK register). Where a GC number is also printed in this model's own manufacturer manual, both sources were checked to agree.

Just want the most common Keston Combi C30 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 25 documented codes

Tap any card for details

F1

Your boiler's water pressure has dropped too low, which has caused it to stop working for safety reasons.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver braided hose with small valves) underneath the boiler.
  2. Slowly open the valve(s) until you hear water entering the system.
  3. Watch the pressure gauge and close the valves tightly once the needle reaches between 1.0 and 1.5 bar.
  4. Reset the boiler by pressing the 'RE' button or turning the dial to 'Reset'.

Parts commonly replaced

  • No parts usually required
  • Filling loop valve
  • Expansion vessel

Related codes

Full explanation for F1
F2

Your boiler has started to light but the flame has gone out unexpectedly, causing the system to stop working for safety.

High DIY-safe £90-350

What to check first

  1. Check that your gas emergency control valve is open and you have credit on your meter.
  2. Check if other gas appliances in your home, like a hob, are working correctly.
  3. Restart the boiler by pressing the reset button for five seconds.

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Spark generator

Related codes

Full explanation for F2
F3

Your boiler has stopped working because the fan that helps push exhaust gases out of your home is not spinning correctly or has lost power.

High Engineer only £250-£450

Parts commonly replaced

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

Related codes

Full explanation for F3
F4

Your boiler has detected that the temperature sensor for your heating water is either broken or poorly connected, preventing the unit from heating up safely.

High Engineer only £120-£220

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring harness / Connector

Related codes

Full explanation for F4
F5

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

High Engineer only £100-£180

Parts commonly replaced

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

Related codes

Full explanation for F5
F6

Your boiler is struggling to work out how hard it needs to fire because the sensor that monitors the outdoor temperature has stopped communicating correctly.

Medium Engineer only £90-180

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Sensor wiring/harness

Related codes

Full explanation for F6
Fan Fault

Your boiler has stopped working because the mechanical part that clears exhaust gases from the system is not spinning or communicating correctly.

High Engineer only £250-£450

Parts commonly replaced

  • Fan Assembly
  • PCB
  • Wiring Harness

Related codes

Full explanation for Fan Fault
Flame Loss

Your boiler was running but the flame went out unexpectedly, causing the system to stop working for safety.

High DIY-safe £120-£350

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas is reaching the property
  2. Check that your gas meter has credit if you use a prepay system
  3. Check the condensate pipe (the white plastic pipe outside) isn't frozen or blocked
  4. Restart the boiler using the Reset button

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve
  • Condensate trap

Related codes

Full explanation for Flame Loss
Flame On Before Gas On

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

High Engineer only £120-350

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Main PCB

Related codes

Full explanation for Flame On Before Gas On
Showing 11–20 of 25
Flow/Return Reversed

Your boiler has detected that the water temperature readings are backward, suggesting a sensor error or that pipework has been connected incorrectly.

High Engineer only £120-250

Parts commonly replaced

  • Return thermistor
  • Flow thermistor
  • Internal pipework seals

Related codes

Full explanation for Flow/Return Reversed
High Mains Voltage

Your boiler has detected that the electricity supply coming into your home is at a dangerously high voltage and has shut down to protect its internal electronics.

High Engineer only £0-250

Parts commonly replaced

  • PCB (Printed Circuit Board)
  • Surge Protector

Related codes

Full explanation for High Mains Voltage
Ignition Lockout

Your boiler has failed to light several times and has shut down for safety, meaning you currently have no heating or hot water.

High DIY-safe £100-£350

What to check first

  1. Check if your gas supply is active by testing another gas appliance like a hob.
  2. Ensure your condensate pipe hasn't frozen if it is cold outside.
  3. Press the Reset button for five seconds to see if the boiler restarts.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Spark generator
  • Detection lead

Related codes

Full explanation for Ignition Lockout
L1

Your boiler has got 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 fully open to allow water flow.
  2. Ensure there is enough pressure in the system by checking the gauge (it should be between 1 and 1.5 bar).
  3. Press the reset button once the boiler has had at least 15 minutes to cool down.

Parts commonly replaced

  • Pump
  • Overheat Thermostat
  • Main Heat Exchanger

Related codes

Full explanation for L1
L2

Your boiler has tried to light several times but cannot establish a flame, meaning your heating and hot water will not work until the issue is resolved.

High DIY-safe £120-350

What to check first

  1. Check that your gas prepayment meter has credit (if applicable).
  2. Ensure the main gas supply valve is open.
  3. Check if other gas appliances in your home, like a hob, are working.
  4. Press the 'Reset' button on the boiler control panel.
  5. In freezing weather, check if the external condensate pipe is frozen and thaw it with warm water.

Parts commonly replaced

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

Related codes

Full explanation for L2
L6

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

Emergency Engineer only £150-£400

Parts commonly replaced

  • Flame sense electrode
  • Gas valve
  • Main PCB

Related codes

Full explanation for L6
Low Mains Voltage

Your boiler has detected that the electricity supply coming into your home is too low for it to operate safely and has shut down to protect its internal parts.

High Engineer only £0-250

Parts commonly replaced

  • Main PCB
  • Voltage Stabiliser

Related codes

Full explanation for Low Mains Voltage
Outside Sensor Fault

Your boiler has lost communication with the sensor that monitors the outdoor temperature, which might make your heating less efficient.

Low Engineer only £90-180

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring harness

Related codes

Full explanation for Outside Sensor Fault
Overheat Lockout

Your boiler has switched itself off because it has detected it is getting too hot, usually caused by a lack of water flow through the system.

High DIY-safe £120-350

What to check first

  1. Check the pressure gauge and top up the water if it is below 1.0 bar
  2. Ensure at least two radiators are fully turned on and their valves are open
  3. Reset the boiler by pressing the reset button for five seconds

Parts commonly replaced

  • Circulating Pump
  • Overheat Thermostat
  • Main Heat Exchanger (System Flush)

Related codes

Full explanation for Overheat Lockout
PCB Fault

Your boiler's main internal computer has stopped working correctly, meaning it can no longer control the heating and hot water functions safely.

High Engineer only £350-£550

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for PCB Fault
Showing 21–25 of 25
Return Thermistor Fault

Your boiler's internal sensor for monitoring water temperature is giving an incorrect reading, meaning the system cannot safely manage your heating.

High Engineer only £90-160

Parts commonly replaced

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

Related codes

Full explanation for Return Thermistor Fault
Room Stat Fault

Your boiler is not turning on because it has lost communication with your wall-mounted thermostat.

High DIY-safe £0-120

What to check first

  1. Locate your wall-mounted room thermostat or programmer.
  2. Replace the batteries with new, high-quality alkaline batteries.
  3. Check that the thermostat is set to a temperature higher than the current room temperature to call for heat.

Parts commonly replaced

  • AA/AAA Batteries
  • Wireless Room Thermostat
  • Thermostat Receiver
Full explanation for Room Stat Fault
Spare PCB Not Set

Your boiler's computer brain has been replaced or reset and needs to be told which specific model it is controlling before it can start working.

High Engineer only £90-180

Parts commonly replaced

  • Main Circuit Board (PCB)

Related codes

Full explanation for Spare PCB Not Set
Too Many Restarts

Your boiler is repeatedly trying to start up but failing, causing it to shut down to protect its internal components.

High Engineer only £120-350

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Valve
  • PCB (Printed Circuit Board)

Related codes

Full explanation for Too Many Restarts
Water Pressure

Your boiler has stopped working because the water pressure inside the system is too low and needs to be topped up.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath the boiler.
  2. Open the small taps on the loop slowly until you hear water flowing and the pressure gauge reaches 1.0 to 1.5 bar.
  3. Close both taps tightly and restart the boiler if necessary.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for Water Pressure
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