Ideal Independent System2 S15 Fault codes & diagnostics

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

System Natural Gas 15 kW 2022-present
GC Number 41-860-06
Download the Ideal Independent System2 S15 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.
23 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
41-860-06 INDEPENDENT SYSTEM2 S15 2022–present
Just want the most common Ideal Independent System2 S15 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

F 0t

Your boiler has lost communication with the wall thermostat, meaning it cannot receive the signal to turn your heating on.

Medium DIY-safe £80-220

What to check first

  1. Check if your wall thermostat has run out of batteries and replace them if necessary.
  2. Ensure the thermostat is turned up higher than the current room temperature.
  3. If using a wireless model, ensure the receiver box is powered on and within range of the thermostat.

Parts commonly replaced

  • Thermostat batteries
  • Room thermostat
  • RF Receiver unit

Related codes

Full explanation for F 0t
F 0U

Your boiler has sensed that the water leaving it is much hotter than the water returning, meaning heat is not moving around your system properly.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves (TRVs) are fully open
  2. Check that the external pump (if visible) is running and not stuck
  3. Ensure there is enough water pressure in the system by checking the pressure gauge
  4. Bleed all radiators to remove trapped air

Parts commonly replaced

  • External central heating pump
  • System filter (cleaning/servicing)
  • Thermistor (Flow/Return sensor)

Related codes

Full explanation for F 0U
F 01

Your boiler has detected that there is not enough water in the system to operate safely, causing it to shut down temporarily.

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 small tap(s) on the filling loop to allow water into the system.
  3. Watch the pressure gauge and close the taps once it reaches 1.5 bar, then restart the boiler.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for F 01
F 02

Your boiler has started an ignition sequence but the flame went out or could not be detected during operation.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas supply is active
  2. Ensure your gas meter has credit if you use a top-up meter
  3. Press and hold the reset button for two seconds

Parts commonly replaced

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

Related codes

Full explanation for F 02
F 03

Your boiler has stopped working because the fan that clears exhaust gases out of the flue is not spinning correctly.

High Engineer only £280-450

Parts commonly replaced

  • Fan motor
  • PCB
  • Air pressure switch

Related codes

Full explanation for F 03
F 04

Your boiler has lost its ability to measure the temperature of the water leaving the unit, causing it to stop working for safety.

High Engineer only £90-160

Parts commonly replaced

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

Related codes

Full explanation for F 04
F 05

Your boiler has lost signal from the sensor that monitors the temperature of the water returning from your radiators, causing it to stop running for safety.

High Engineer only £90-160

Parts commonly replaced

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

Related codes

Full explanation for F 05
F 06

Your boiler has lost connection with the sensor that measures the temperature outdoors, which helps it run more efficiently.

Low Engineer only £90-160

Parts commonly replaced

  • External weather compensation kit
  • Outside sensor wiring

Related codes

Full explanation for F 06
F 07

Your boiler is not receiving enough electrical power from the grid to operate safely and has shut down as a precaution.

High Engineer only £0-250

Parts commonly replaced

  • Main PCB
  • Voltage Regulator

Related codes

Full explanation for F 07
F 09

Your boiler's main control board has suffered an internal electronic failure and is no longer able to manage the heating system.

High Engineer only £280-450

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for F 09
Showing 11–20 of 25
F 10

Your boiler is unable to create the spark needed to light the flame, which means your heating and hot water will not work.

High Engineer only £140-£240

Parts commonly replaced

  • Spark Generator
  • Ignition Lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F 10
F 12

Your boiler has stopped working because the internal pump that moves hot water through your pipes is jammed or stuck.

High Engineer only £180-350

Parts commonly replaced

  • Central Heating Pump
  • Pump Head

Related codes

Full explanation for F 12
F 13

Your boiler's internal pump has stopped circulating water, which means the system cannot deliver heat to your radiators.

High Engineer only £250-£450

Parts commonly replaced

  • Circulation Pump
  • Pump head
  • Wiring harness

Related codes

Full explanation for F 13
F 14

Your boiler has detected that the electricity supply from the national grid is currently too high for it to operate safely.

High Engineer only £0-350

Parts commonly replaced

  • Main Printed Circuit Board
  • Voltage Regulator

Related codes

Full explanation for F 14
F 15

Your boiler has detected a problem with the internal pump, meanining it cannot move heated water around your system to warm your radiators.

High Engineer only £220-£380

Parts commonly replaced

  • Central heating pump
  • Internal pump wiring harness

Related codes

Full explanation for F 15
F 16

Your boiler's gas valve is not communicating correctly with the control board, meaning the unit cannot safely manage the flow of fuel to create heat.

High Engineer only £220-£380

Parts commonly replaced

  • Gas Valve
  • Control Lead / Wiring Harness
  • Main PCB

Related codes

Full explanation for F 16
FN

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

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances in your home (like a hob) are working to confirm gas supply.
  2. Check if your gas meter has credit or if the emergency control valve is open.
  3. Press the reset button once to see if the boiler restarts.

Parts commonly replaced

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

Related codes

Full explanation for FN
L 0C

Your boiler is repeatedly attempts to start up and failing, causing it to lock itself out for safety after five unsuccessful tries.

High Engineer only £120-£350

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Electrode
  • Gas Valve
  • Fan

Related codes

Full explanation for L 0C
L 0d

Your boiler has stopped working because it cannot safely get rid of waste gases or drainage water, often due to a blockage in the external pipes or a frozen drain.

High DIY-safe £80-220

What to check first

  1. Check the white plastic pipe outside for ice or blockages and clear if necessary.
  2. If the pipe is frozen, gently pour warm (not boiling) water over it to thaw it.
  3. Press the Reset button on the boiler control panel once the blockage is cleared.

Parts commonly replaced

  • Condensate trap
  • Printed Circuit Board (PCB)
  • Flue Fan

Related codes

Full explanation for L 0d
L 01

Your boiler has switched itself off because it has become too hot internally and needs to cool down for safety.

High DIY-safe £120-£350

What to check first

  1. Check that your radiator valves are open and not turned off.
  2. Ensure there is enough water pressure in the system by checking the pressure gauge (usually between 1 and 1.5 bar).
  3. Press the reset button on the front of the boiler once it has cooled down.

Parts commonly replaced

  • Circulating Pump
  • Overheat Thermostat
  • Main Heat Exchanger (if blocked)

Related codes

Full explanation for L 01
Showing 21–25 of 25
L 02

Your boiler has tried to light several times but cannot establish a flame, usually because the gas supply is interrupted or a component is faulty.

High DIY-safe £100-£350

What to check first

  1. Check if your gas meter has credit (if on a prepayment meter)
  2. Check if other gas appliances, like a hob, are working to confirm gas is flowing
  3. Press the 'Reset' button on the front of the boiler for five seconds
  4. If it is freezing cold outside, check if your white condensate pipe is frozen and thaw it with warm water

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Spark Leads
  • Printed Circuit Board (PCB)

Related codes

Full explanation for L 02
L 04

Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, causing it to stop running for safety.

High Engineer only £90-180

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness
  • Control Board (PCB)

Related codes

Full explanation for L 04
L 05

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 £90-160

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring Harness

Related codes

Full explanation for L 05
L 06

Your boiler thinks a flame is present before the gas has even started flowing, which is a safety conflict that prevents the system from starting.

High Engineer only £120-£350

Parts commonly replaced

  • Ionisation probe
  • Gas valve
  • Main PCB

Related codes

Full explanation for L 06
LN

Your boiler has lost its flame while running and has shut down as a safety precaution.

High DIY-safe £100-350

What to check first

  1. Check that your gas supply is active and that other gas appliances are working
  2. Check that the condensate pipe (the plastic pipe running outside) isn't frozen if it is very cold weather
  3. Press the Reset button on the front of the boiler

Parts commonly replaced

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

Related codes

Full explanation for LN
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