Keston F2 Fault Code: Causes, Fixes & Repair Costs
What does the Keston F2 fault code mean?
The F2 code on a Keston boiler signals a flame loss event — the boiler attempted to burn but the flame either failed to establish or dropped out during operation. Depending on how persistent the problem is, the boiler will either pause and retry automatically (a block) or lock out completely and wait for you to press the Reset button. Switching the boiler off and on at the programmer or mains will not clear an F2 lockout; the Reset button on the boiler itself must be used. Keston's twin-flue design means the flue system is a particularly important area to investigate, as dust and spider webs can accumulate in places not seen on conventional boilers.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Frozen or blocked condensate pipe Common
During cold weather, the plastic condensate pipe that carries acidic waste water outside can freeze solid. When it does, the boiler shuts down and an F2 flame-loss code can follow because the boiler cannot complete a firing cycle. This is one of the most frequent causes of sudden F2 faults in winter.
- Gas supply interruption or low inlet pressure Common
If the gas pressure reaching the Keston's gas valve falls outside the acceptable range — below 18 mbar for natural gas or below 31.5 mbar for LPG — the burner cannot sustain a stable flame and trips on F2. Check whether other gas appliances in the property are working normally. If they are not, the issue lies with the supply rather than the boiler.
- Worn or dirty ignition/flame detection electrode Common
The same electrode that sparks to light the burner also confirms the flame is present. Carbon deposits, corrosion, or physical cracking can cause it to misread or fail to detect the flame altogether, producing repeated F2 faults even when gas supply and pressure are correct.
- Flue obstruction Sometimes
Keston's twin-flue system draws combustion air in through one pipe and expels exhaust through another, giving birds, insects, and debris two potential entry points. A partial blockage can starve the burner of air or cause combustion gases to recirculate, leading to an unstable flame and an F2 error.
- Faulty or sticky gas valve Sometimes
A gas valve that sticks partially closed will restrict fuel flow, starving the flame. Conversely, a valve that does not seat properly can allow residual gas to pass and trigger false flame signals. Either scenario can produce intermittent or persistent F2 faults.
- Wiring fault or PCB failure Rare
Loose, corroded, or damaged wiring between the gas valve and the PCB can prevent the control board from correctly managing the ignition sequence. If wiring continuity checks out but the fault persists, the PCB itself may need replacement. This is a less common cause but is included in Keston's own engineer diagnostic pathway.
How to fix it
- Check other gas appliances in the property DIY safe
Turn on a gas hob ring or check your gas cooker. If nothing else works either, the problem is almost certainly with the gas supply itself — check that the meter isolation valve is fully open and, if you have a prepayment meter, that it has credit. If your neighbours are also without gas, contact your gas network operator.
- Inspect the condensate pipe for freezing DIY safe
Locate the plastic waste pipe (usually 21.5 mm or 32 mm diameter) that exits the boiler and runs to an external drain or soakaway. In freezing weather, feel along its length for a solid, icy section. You can thaw it carefully by pouring warm (not boiling) water over the frozen section or wrapping it in a hot-water bottle. Never use a naked flame.
- Reset the boiler — but only once or twice DIY safe
Once any obvious issue is resolved, press and hold the Reset button on the boiler fascia for a few seconds until the display changes. If the boiler fires successfully, monitor it over the next hour. If it trips F2 again within a short time, stop resetting — repeated resets without fixing the root cause can cause further damage and will not resolve the underlying fault.
- Gas Safe engineer: verify gas inlet pressure at the valve Gas Safe engineer
A registered engineer will connect a manometer to the gas valve test point and confirm that inlet pressure sits within specification — 18–22 mbar for natural gas, 31.5–37.6 mbar for LPG — while the boiler is firing. Pressure outside these limits points to a supply or regulator issue that must be resolved before further diagnostics.
- Gas Safe engineer: inspect and test the ignition and flame detection electrode Gas Safe engineer
The engineer will examine the electrode for cracking, carbon tracking, and corrosion, and will disconnect the in-line connector to test signal continuity and integrity. A deteriorated electrode should be cleaned or replaced along with its wiring harness if the insulation is degraded.
- Gas Safe engineer: check the twin-flue system for obstructions Gas Safe engineer
Keston's concentric or twin-pipe flue arrangement requires a thorough inspection at both the terminal and along the duct run. The engineer should check for bird nests, insect debris, and soot build-up that can restrict airflow and destabilise combustion.
- Gas Safe engineer: test wiring continuity between gas valve and PCB Gas Safe engineer
Using a multimeter, the engineer checks for continuity and correct resistance in the wiring loom connecting the gas valve to the control board. Any broken, chafed, or corroded wire must be repaired or replaced. If the loom is sound but the fault persists, the spark generator and harness should be substituted.
- Gas Safe engineer: replace the PCB if all other components test satisfactory Gas Safe engineer
If gas pressure, the electrode, the flue, and the wiring harness all check out correctly, the PCB is the next likely culprit. Keston parts and technical support remain available through Ideal (Keston's parent group) and the Keston technical helpline. Always confirm the replacement board is mapped to the correct boiler variant.
- Call a Gas Safe registered engineer Gas Safe engineer
If the above DIY checks have not resolved the F2 fault, you need a Gas Safe registered engineer with experience on Keston appliances. Keston's twin-flue and modulating-pump design means not every heating engineer is familiar with the system — ask specifically whether they have worked on Keston boilers before booking, or use the engineer finder at keston.co.uk.
Parts you may need
- Ignition and flame detection electrode · from £45
- Spark generator and ignition harness · from £65
- Gas valve (natural gas or LPG variant) · from £130
- PCB (control board) · from £200
- Condensate pipe repair kit / external insulation lagging · from £15
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Can I reset the Keston F2 fault myself?
Yes — pressing the Reset button on the boiler is a safe homeowner action, and it is worth trying once or twice after checking the condensate pipe and gas supply. However, if the F2 code keeps returning within minutes or hours, stop resetting and call an engineer. Repeated resets without fixing the root cause will not help and can occasionally mask a more serious problem.
Is the F2 code always caused by a gas problem?
Not necessarily. While gas supply and pressure are the most common culprits, a frozen condensate pipe, a failing ignition electrode, or a flue obstruction can all produce an F2 flame-loss fault. An engineer will work through the full diagnostic sequence rather than assuming the gas valve is at fault.
Why does my Keston F2 fault keep coming back?
Recurring F2 faults usually point to either intermittent low gas pressure, a partially worn electrode that sometimes detects the flame and sometimes does not, or a partially obstructed flue. Keston's twin-flue design is particularly prone to spider webs and dust accumulation in the flue terminal, so if the fault clears on reset but returns regularly, insist the engineer checks the full flue run during their visit.
How much will an F2 repair cost on a Keston boiler?
Most F2 repairs fall in the range of £120–£380, covering an engineer's call-out and the most commonly replaced components — the electrode, spark generator, or wiring harness at the lower end, and a gas valve replacement at the higher end. PCB replacement typically costs £250–£450 including labour and is less common; if a PCB and gas valve are both needed on a boiler over 12–15 years old, it is worth comparing that cost against a new boiler replacement, since modern condensing boilers are substantially more efficient.