Ideal i-mini C24 / C30 Fault codes & diagnostics

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

Combi Natural Gas 24-30 kW 2016-present
Choose your boiler type

The i-mini C24 / C30 range comes in 2 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

C24 18 codes · 1 GC number C30 18 codes · 1 GC number
Each model has its own manual A family spans several manuals, one per model or generation. Pick your model above and its page has the manual its fault codes were read from.

The fault code is already on your display

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.
2 Emergency 14 High 1 Medium 1 Low
Just want the most common Ideal i-mini C24 / C30 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 18 documented codes

Tap any card for details

dU

Your boiler has paused because a valve is stuck halfway, which usually happens during a specific service check or test mode.

Medium DIY-safe Shared across the i-mini range

What to check first

  1. Locate the hot water and heating temperature knobs on the front of your boiler.
  2. Rotate both knobs fully clockwise (all the way to the right).
  3. Turn the boiler power off at the fused spur, wait a few seconds, and turn it back on to restart the system.
Full explanation for dU
F1

Your boiler has temporarily stopped working because the water pressure inside the system has dropped too low.

High DIY-safe Shared across the i-mini range

What to check first

  1. Locate the filling loop underneath your boiler, which usually consists of a silver braided hose and one or two small valves.
  2. Open the valves slowly to allow water into the system, and watch the pressure gauge on the front panel until the needle sits between 1.0 and 1.5 bar.
  3. Firmly close both valves and check the gauge holds steady, then reset your boiler if it does not restart automatically.

Parts commonly replaced

  • Pressure relief valve
  • Pressure sensor
  • Automatic air vent

Related codes

Full explanation for F1
F2 or Fn or Ln

Your boiler has detected that the burner has gone out or failed to stay lit while trying to heat your home.

High Engineer only £100-£250 Shared across the i-mini range

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensor

Related codes

Full explanation for F2 or Fn or Ln
F3

Your boiler has stopped working because the internal fan that safely clears combustion gases is not running properly.

High Engineer only £150-300 Shared across the i-mini range

Parts commonly replaced

  • Flue fan
  • Air pressure switch
  • Printed circuit board (PCB)

Related codes

Full explanation for F3
F4 OR L4

Your boiler has temporarily stopped working because it cannot accurately measure the temperature of the water flowing through the system due to a faulty sensor.

High Engineer only £120-£200 Shared across the i-mini range

Parts commonly replaced

  • Flow thermistor
  • NTC sensor wiring harness

Related codes

Full explanation for F4 OR L4
F5 OR L5

Your boiler has temporarily stopped working because it cannot accurately read the temperature of the water returning to it.

High Engineer only £100-£180 Shared across the i-mini range

Parts commonly replaced

  • Return thermistor
  • Wiring harness

Related codes

Full explanation for F5 OR L5
F6

Your boiler has lost connection to its outdoor temperature sensor, but it should still continue to heat your home using its internal settings.

Low Engineer only £90-150 Shared across the i-mini range

Parts commonly replaced

  • Outside temperature sensor
  • Sensor wiring

Related codes

Full explanation for F6
F7

Your boiler has detected that the electricity supply coming into your home from the mains is too low to run it safely.

High Engineer only Shared across the i-mini range

Related codes

Full explanation for F7
F9 or L9 or F8 or L8

Your boiler's main electronic brain has lost its programming or failed, meaning it doesn't know how to run your heating safely.

High Engineer only £200-450 Shared across the i-mini range

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

Full explanation for F9 or L9 or F8 or L8
FA

Your boiler has temporarily stopped working because it has detected an issue with the temperature sensors measuring the water flow.

High Engineer only £100-£200 Shared across the i-mini range

Parts commonly replaced

  • Flow thermistor
  • Return thermistor
  • Wiring harness

Related codes

Full explanation for FA
FU

Your boiler has temporarily stopped working because water is not flowing through the system properly, causing it to overheat internally.

High DIY-safe £90-250 Shared across the i-mini range

What to check first

  1. Check that the valves underneath the boiler (flow and return isolation valves) are fully open.
  2. Check that at least some of your radiators around the house have their valves fully open.
  3. Check your boiler pressure gauge to ensure it is sitting between 1 and 1.5 bar, and top it up using the filling loop if necessary.

Parts commonly replaced

  • Central heating pump
  • Automatic bypass valve
  • Heat exchanger

Related codes

Full explanation for FU
L 2

Your boiler has stopped working because it failed to ignite after five attempts and has safely locked itself out.

High Engineer only £100-£250 Shared across the i-mini range

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for L 2
L1

Your boiler has temporarily shut itself down because the water inside it has become dangerously hot.

Emergency Engineer only £150-350 Shared across the i-mini range

Parts commonly replaced

  • NTC thermistor (temperature sensor)
  • Automatic bypass valve
  • Circulating pump
  • Heat exchanger

Related codes

Full explanation for L1
L2

Your boiler has stopped working because it tried to start up several times but failed to detect a flame.

High Engineer only £100-250 Shared across the i-mini range

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for L2
L4 or F4

Your boiler has temporarily stopped working because it is unable to accurately measure the temperature of the water flowing inside it.

High Engineer only £90-180 Shared across the i-mini range

Parts commonly replaced

  • Flow Thermistor
  • NTC Sensor

Related codes

Full explanation for L4 or F4
L5 or F5

Your boiler has temporarily stopped working because it cannot accurately measure the temperature of the water returning to it.

High Engineer only £120-£220 Shared across the i-mini range

Parts commonly replaced

  • Return thermistor (temperature sensor)
  • Wiring harness

Related codes

Full explanation for L5 or F5
L6

Your boiler has detected a flame inside when it shouldn't be burning, so it has safely locked itself out to prevent any risk.

Emergency Engineer only £150-350 Shared across the i-mini range

Parts commonly replaced

  • Gas valve
  • Main printed circuit board (PCB)
  • Electrodes

Related codes

Full explanation for L6
LC

Your boiler has temporarily locked itself out because it kept trying and failing to restart too many times in a short period.

High Engineer only £90-250 Shared across the i-mini range

Parts commonly replaced

  • Ignition lead
  • Electrodes
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for LC
Boiler cover options

Avoid the next call-out bill

Monthly cover means no unexpected bills when your boiler breaks down.

Approved partner
Domestic Appliance Guard
Unlimited call-outs, 24/7 helpline, no price-hike promise
Get a quote →

Affiliate links. We may earn a commission at no extra cost to you.