Keston F3 Fault Code: Causes, Fixes & Repair Costs
What does the Keston F3 fault code mean?
The F3 fault code on a Keston boiler indicates a problem with the flue fan. Specifically, the boiler's PCB has detected that the fan is either not running at all or is failing to reach the speed required for safe operation. Because the fan is responsible for drawing combustion gases out through the flue, the boiler will not attempt to ignite without confirming the fan is working correctly — so when F3 appears, the boiler enters lockout and shuts down entirely. Note that the F3 code is associated primarily with the Keston S30 system boiler; if you have a Keston C36 combi, a similar fan-related fault will appear as E05 rather than F3.
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
- Worn or seized fan motor bearings Common
Over time, the small bearings inside the fan motor wear down, causing the fan to spin too slowly or stop altogether. A boiler that has been making a grinding or rattling noise before the F3 appeared is a strong sign that bearing wear is the culprit. This is the most frequent reason for this fault code on Keston appliances.
- Blocked venturi Common
The venturi is a narrow air-mixing component that sits close to the fan. Spiders and insects are particularly attracted to it, and a partial blockage can restrict airflow enough to prevent the fan from reaching its target speed. The boiler's PCB then registers the fault even though the fan motor itself may be perfectly healthy.
- Faulty air pressure switch or hose Sometimes
The air pressure switch monitors the draught created by the fan. If the switch sticks in the open or closed position, or if the rubber hose connecting it to the fan assembly has split or become blocked with condensation, the PCB receives a false reading and triggers the F3 lockout even when the fan is actually spinning normally.
- Wiring fault between fan and PCB Sometimes
Damaged, corroded, or loose wiring in the harness that carries the fan speed signal to the PCB can cause the board to register zero or erratic speed readings. Moisture ingress — more common in older Keston boilers whose electrical components are less well-sealed — can accelerate wiring degradation.
- Blocked or partially obstructed flue Sometimes
Keston boilers use a twin-flue design. If either the inlet or outlet flue terminal becomes blocked — by debris, bird nesting material, or ice in freezing conditions — back pressure builds up and the air pressure switch detects that the draught is wrong, which can produce a fan-related lockout.
- PCB failure Rare
In rare cases the PCB itself loses the ability to read the fan speed signal correctly, or the component on the board that drives the fan fails. This tends to be a diagnosis of exclusion — reached only after the fan, wiring, and pressure switch have been ruled out.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button on the boiler's control panel as described in your user manual — typically for around three seconds. Simply switching the boiler off and back on at the mains will not clear a lockout on Keston appliances. If the boiler fires up and runs normally, monitor it closely. If F3 returns within a short time, do not keep resetting repeatedly, as this can mask a developing fault.
- Check that your gas supply is live DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier rather than an engineer, as the issue is with the supply rather than the boiler.
- Inspect the flue terminals for obvious blockages DIY safe
Look at the external flue outlet and inlet terminals (usually on an external wall or through the roof). Check for visible obstructions such as bird nests, leaves, or ice. Do not attempt to dismantle any flue components — if you can see a blockage you cannot safely clear from outside, report it to a Gas Safe engineer.
- Have a Gas Safe engineer inspect the fan assembly Gas Safe engineer
An engineer will check the fan motor, measure its running speed, inspect the venturi for blockages, and test the fan's electrical connections. Keston boilers are regarded as complex appliances due to their twin-flue system and modulating pump, so it is worth specifically asking whether the engineer has experience with Keston products before booking.
- Have the air pressure switch and hose tested Gas Safe engineer
Using a multimeter and a pressure gauge, the engineer will verify that the air pressure switch is operating within specification and that the connecting rubber hose is free of splits, blockages, or trapped condensation. A faulty switch or perished hose is replaced at this stage.
- Have any damaged wiring or moisture ingress addressed Gas Safe engineer
If the engineer finds corroded or degraded wiring in the fan circuit, the affected sections will need to be dried or replaced. Where moisture is traced back to an internal leak, the source must be repaired at the same time — simply replacing wiring without fixing a leak will result in the problem returning.
- Replace the fan motor if required Gas Safe engineer
If the motor itself has failed or its bearings are beyond use, the engineer will fit a replacement fan unit. Only genuine or approved equivalent parts should be used on Keston boilers given the specific airflow requirements of the twin-flue system.
- PCB investigation as a last resort Gas Safe engineer
If the fan, wiring, venturi, and pressure switch have all been checked and found to be in good condition, the PCB is the remaining suspect. Board replacement or repair should only be carried out by a qualified engineer, and on an older boiler it is worth weighing the cost of a new board against the cost of a full boiler replacement.
Parts you may need
- Keston fan motor assembly · from £130
- Air pressure switch · from £35
- Venturi / air-gas mixing unit · from £45
- Pressure switch connecting hose · from £12
- Fan wiring harness · from £30
- PCB (control board) · from £280
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 £225–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Keston F3 fault myself?
The only steps a homeowner can safely take are resetting the boiler (once or twice at most), checking that the gas supply is live, and looking for obvious external flue blockages. Everything else — testing the fan, pressure switch, wiring, or PCB — must be carried out by a Gas Safe registered engineer. Working on boiler components without registration is illegal in the UK and potentially dangerous.
Why does my Keston boiler show F3 but the fan sounds like it is running?
This is actually quite common. The fan may be spinning but not reaching its required speed — often because of a blocked venturi restricting airflow, or worn bearings causing it to run slowly. Alternatively, the fan could be running perfectly but a faulty air pressure switch or a split hose is sending a false signal to the PCB. In both cases the boiler correctly locks out as a safety precaution. An engineer will measure the actual fan speed and test the pressure switch circuit to find the real cause.
How much does it cost to repair a Keston F3 fault in the UK?
For most households, the repair involves either clearing a blocked venturi, replacing the fan motor, or fitting a new air pressure switch. All-in costs (parts plus a Gas Safe engineer's labour) typically fall in the range of £225 to £350. If the PCB turns out to be the root cause, costs can rise to £400–£500 or more, which is worth bearing in mind on older boilers — at that point, a replacement boiler may be a more economical long-term choice.
Keston boilers are supposed to be complex — do I need a specialist engineer?
Keston appliances use a twin-flue design and a modulating pump, which differs from many standard combi boilers. While any Gas Safe registered engineer is legally permitted to work on them, not all are familiar with the specifics. It is worth asking your engineer upfront whether they have worked on Keston boilers before, particularly for a fan-related fault where correct airflow settings are critical to safe operation after the repair.