Ideal F4 Fault Code
The F4 fault means the boiler established a flame but the ionisation probe detected it was lost during the heating cycle. Unlike F1 — which is a failure to ignite — F4 means the boiler lit, then lost its flame mid-operation.
What is the Ideal F4 fault code?
The Ideal F4 fault is triggered when the ionisation probe signals flame loss during an active heating or hot water cycle. The boiler established a flame successfully (passing the initial ignition check that would have triggered F1), but the probe then detected that the flame went out. This distinction matters diagnostically — F1 points to ignition components, while F4 more often points to combustion stability, gas supply, or a probe that is degrading rather than completely failed.
What causes the F4 fault?
A partially degraded probe may detect the initial flame but lose signal as combustion conditions change under load. Cleaning may extend its life, but replacement is often the more reliable fix.
Pressure fluctuations in the gas network — particularly during peak demand — can momentarily reduce gas supply below what the burner needs to sustain a flame.
A partially blocked flue creates backpressure that disrupts combustion air during the heating cycle, extinguishing the flame.
A blocked condensate trap restricts the flue gas path and can disturb combustion stability, causing intermittent flame loss.
A failing PCB can misread ionisation signals and generate false F4 codes even when combustion is stable. Diagnosed by elimination after all other components pass.
On Logic+ boilers in hard water areas, limescale on the heat exchanger can alter flow dynamics and disrupt the flame during the heating cycle.
What you can safely check yourself
Check that other gas appliances work normally. If the gas hob is also intermittent, the supply is the problem — call your gas supplier.
In cold weather, thaw a frozen condensate pipe with warm water poured over the external section, then attempt a reset.
Hold the reset button for 3 seconds. If F4 appears on the next heating cycle, call an engineer — do not keep resetting.
Do not reset the boiler more than once if the fault code returns immediately. Repeated resets without diagnosis can mask a worsening fault.
What needs a Gas Safe engineer
- Clean or replace the ionisation probe
- Inspect the burner for carbon deposits
- Measure gas inlet pressure and check for supply fluctuations
- Inspect the flue terminal for partial blockage
- Clear any blocked condensate trap
- Check the PCB ionisation input circuit if all other components test correctly
F4 from ionisation probe deposits is more frequent in London's hard water zones. South London boroughs — Wandsworth, Merton, Lewisham, Southwark, Lambeth, Greenwich, and Bromley — are all affected by Thames Water's calcium-heavy supply. Annual servicing with a probe clean keeps F4 faults rare on Ideal Logic+ and Vogue boilers in these areas.
How much does a F4 repair cost?
Costs below are UK averages. Reion provides a fixed quote after the £80 diagnostic visit — no obligation to proceed.
How to reset a Ideal boiler showing F4
Check that other gas appliances work normally.
In winter, check and thaw the condensate pipe if frozen.
Hold the reset button for 3 seconds.
If F4 appears on the next heating cycle, do not reset again — call a Gas Safe engineer.
Note whether F4 appears immediately or only after the boiler has been running for several minutes, and tell the engineer.
Ionisation probe and burner cleaning are among the lower-cost Ideal repairs. PCB replacement is more significant — on an older boiler with other age-related faults, Reion will give an honest repair-vs-replace assessment at the diagnostic visit.
Common questions about the Ideal F4 fault
What is the difference between an Ideal F1 and F4 fault?
Why does my Ideal boiler only show F4 in cold weather?
Can an Ideal F4 fault fix itself?
How much does an Ideal F4 repair cost in London?
Is an Ideal F4 fault dangerous?
Other Ideal fault codes
Need a Ideal engineer in London?
Reion is Gas Safe registered (919881), based in South London, and available 24/7. Fixed price after the diagnostic visit — no hidden charges.
