Keston System S30 Fault codes & diagnostics

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

System Natural Gas 30 kW 2016 onwards
GC Number 41-930-45
Download the Keston System S30 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.
19 High 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
41-930-45 KESTON SYSTEM S30 2016–present
Just want the most common Keston System S30 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 20 documented codes

Tap any card for details

F1

Your boiler has detected that the water pressure in the system is too low to run safely and has shut down as a precaution.

High DIY-safe £0-120

What to check first

  1. Locate the metal braided filling loop underneath or near the boiler.
  2. Slowly open the small handles on the 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
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for F1
F2

Your boiler has lit correctly but is struggling to keep the flame burning, causing it to shut down for safety.

High DIY-safe £120-350

What to check first

  1. Check that your gas emergency control valve is fully open
  2. If you have a prepayment meter, check that you have sufficient credit
  3. Try resetting the boiler by pressing the 'Restart' button

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Spark generator
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F2
F3

Your boiler's internal fan is not spinning or communicating correctly, preventing the system from safely clearing gases so it has shut down for safety.

High Engineer only £250-£450

Parts commonly replaced

  • Extraction Fan
  • Fan Wiring Harness
  • Main PCB

Related codes

Full explanation for F3
F4

Your boiler has stopped working because it cannot detect water moving through the system correctly, which is usually caused by a faulty sensor or a blockage.

High Engineer only £120-£250

Parts commonly replaced

  • Flow Thermistor
  • Return Thermistor
  • Pump

Related codes

Full explanation for F4
F5

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning from your radiators, causing the system to stop working for safety.

High Engineer only £120-220

Parts commonly replaced

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

Related codes

Full explanation for F5
F6

Your boiler has lost connection with the sensor that measures the temperature outdoors, which may cause your heating to run less efficiently.

Low Engineer only £90-180

Parts commonly replaced

  • External weather compensation sensor
  • Sensor wiring

Related codes

Full explanation for F6
F7

Your boiler has stopped working because the electrical supply from the grid or your home's wiring has dropped below the level required for the unit to run safely.

High Engineer only £100-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Voltage Regulator

Related codes

Full explanation for F7
F8

Your boiler's internal computer has lost its settings or experienced a failure, meaning it cannot currently operate because it does not know how to manage its own functions.

High Engineer only £250-450

Parts commonly replaced

  • Main Control PCB
  • Configuration Key

Related codes

Full explanation for F8
F9

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

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for F9
Fn

Your boiler has detected that the flame inside has gone out unexpectedly while it was trying to heat your home or water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

Full explanation for Fn
Showing 11–20 of 20
FU

Your boiler is overheating because water is not flowing through your heating system properly to carry the heat away.

High DIY-safe £120-£250

What to check first

  1. Check all your radiator valves are turned fully open and not stuck shut.
  2. Ensure your central heating system pressure is at the correct level (typically 1.0–1.5 bar on the gauge).
  3. Check under the boiler to ensure the red isolation valves on the pipework are turned into the open position.

Parts commonly replaced

  • Central heating pump
  • Flow/Return sensors
  • Diverter valve

Related codes

Full explanation for FU
L1

Your boiler has stopped working because it has detected that the water inside it is getting too hot.

High DIY-safe £100-£350

What to check first

  1. Check that all radiator valves are open to ensure water can circulate.
  2. Ensure there is enough water pressure by checking the gauge is between 1 and 1.5 bar.
  3. Press the 'Reset' button on the front of the boiler.

Parts commonly replaced

  • Circulation Pump
  • Heat Exchanger (Cleaning or Replacement)
  • Thermistor

Related codes

Full explanation for L1
L2

Your boiler is failing to keep a flame lit, which means the system has shut itself down for safety and you will not have any heating or hot water.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances in your home (like a hob) are working to ensure gas is reaching the property.
  2. Ensure your gas meter has credit if you have a prepaid meter.
  3. Check if the white plastic condensate pipe leading outside has frozen during cold weather and thaw it with warm water.
  4. Press the 'Reset' button on the front of the boiler for five seconds.

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Ignition lead
  • Spark generator

Related codes

Full explanation for L2
L4

Your boiler has detected that it cannot accurately measure the temperature of the water flowing through it, so it has shut down to prevent damage.

High Engineer only £150-250

Parts commonly replaced

  • Flow / Return Thermistor
  • Wiring harness / Cable assembly

Related codes

Full explanation for L4
L5

Your boiler is unable to read the temperature of the water returning to it, so it has safely shut down to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness

Related codes

Full explanation for L5
L6

Your boiler thinks it sees a flame even though it hasn't actually started the ignition process yet.

High Engineer only £120-280

Parts commonly replaced

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

Related codes

Full explanation for L6
L8

Your boiler's computer system has lost its internal settings, or the main control unit itself has developed an internal electrical fault and can no longer operate.

High Engineer only £250-450

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for L8
L9

Your boiler's computer system has lost its internal settings, preventing it from knowing how to safely operate your heating and hot water.

High Engineer only £200-450

Parts commonly replaced

  • Main PCB
  • Interface board

Related codes

Full explanation for L9
LC

Your boiler is repeatedly shutting down and restarting itself too frequently, so it has locked out to protect the system from potential damage.

High Engineer only £150-350

Parts commonly replaced

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

Related codes

Full explanation for LC
Ln

Your boiler has attempted to ignite several times but is unable to sustain a flame, causing it to shut down for safety.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

Full explanation for Ln
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