Overview
On a Keston S30 system gas boiler the F9 fault indicates a problem with the printed circuit board (PCB) or its related electronics. The PCB is the boiler’s control brain: it monitors sensors, controls the gas valve, fan and pump, and runs safety checks. When the board detects an internal fault or inconsistent signals it will lock the boiler out and display F9 to prevent unsafe operation. Severity ranges from a temporary electronic glitch to a permanent PCB failure. Because the PCB controls safety functions, the boiler will usually stop working or run in a restricted mode until the fault is cleared. Some simple faults can be cleared by a safe restart, but repeated or persistent F9 faults generally need a qualified heating engineer. Replacing or repairing the PCB, diagnosing wiring, or confirming low mains voltage are tasks for a Gas Safe registered engineer (or registered installer in your country). If the boiler is under warranty you should contact Keston or your installer before any invasive work.
Possible Cause: Printed circuit board (PCB) Fault
Troubleshooting Steps
Safety precautions:
- Do not attempt gas or mains electrical repairs unless you are a qualified Gas Safe registered engineer or equivalent. Working on live mains or gas components can be lethal and is illegal for unqualified persons.
- If you need to open the boiler for any visual check, first isolate the electricity at the consumer unit and switch the boiler off. Allow the appliance to cool and do not touch any live terminals.
Initial checks a homeowner can do (safe, non-invasive):
1. Record the exact display and any other codes or LEDs visible. Take a photo of the fault for the engineer.
2. Try a single controlled restart using the boiler RESTART button or the procedure in the user manual. If the fault clears and does not return, monitor the boiler closely; if it comes back, stop attempts to restart and call an engineer.
3. Confirm the boiler has a stable mains supply and other house electricals are working. If you suspect low mains voltage contact your electricity supplier or an electrician.
4. Check boiler pressure on the gauge. Although F9 on the S30 is listed as a PCB fault, low or fluctuating pressure can cause other faults; if pressure is below the normal range (typically 1–2 bar) you can re-pressurise using the filling loop following the manual. Do not leave the system over-pressurised.
5. Inspect around the boiler externally for obvious water leaks, corrosion, burn marks, or a damp smell. If you see water on or near the boiler, isolate the boiler electrically and call a Gas Safe engineer immediately.
6. Check external controls: make sure the programmer/timers, room thermostat and hot water cylinder thermostat are calling for heat when expected. Faulty external controls can cause unusual behaviour and confusing codes.
Specific diagnostic and fix steps you may perform safely (visual, non-electrical):
1. With the boiler isolated from electricity, remove the outer casing only if you are comfortable and it is straightforward to do so. Do not touch internal components unless qualified.
2. Visually inspect the PCB area for signs of water ingress, corrosion, burnt components, loose connectors, or broken wires. Take clear photos to show an engineer. If you find moisture or corrosion do not attempt to dry or repair the board yourself beyond documenting the condition.
3. Check wiring looms and connector plugs are seated (visual only). Do not probe live terminals. If a plug looks loose when power is off you can gently reseat it, but do not force or solder connections.
4. Look for any blown fuses in the boiler’s PCB fuse holders or the consumer unit. If a domestic fuse at the consumer unit is tripped, you can reset it, but if a boiler internal fuse is blown, do not replace it with a different rating or continue to run the boiler. Note the blown fuse and call an engineer.
5. If the boiler manual or display mentions an unconfigured PCB or BCC activation fault, contact Keston first if the appliance is under warranty. Unconfigured boards require manufacturer or authorised engineer setup.
When to call a professional and what they will do:
- Call a Gas Safe registered engineer (or registered installer in your country) if the F9 fault persists after a single safe restart, if you find signs of water damage or burnt components, if internal fuses have blown, or if you are unsure about any check above.
- A qualified engineer will safely isolate the appliance, measure mains voltage, perform electrical and component tests, read detailed diagnostic data from the PCB, and inspect sensors, wiring and associated components. If the PCB is faulty they will advise on replacement and re-configuration, test the new board, and run safety checks and a full operational test. They will also investigate root causes such as leaks, condensation, or over-voltage that could damage a new PCB.
Other important notes:
- Repeated resets are not a safe long-term solution; recurring F9 faults almost always need professional diagnosis.
- If the boiler is under warranty contact Keston or your installer before any parts are replaced.
- Document fault occurrences, any other error codes, and the boiler serial number to speed diagnosis when the engineer arrives.
- Do not attempt to repair or replace the PCB yourself unless you are an appropriately qualified engineer. Replacing PCBs and performing gas safety checks require specialist tools, knowledge and certification.
If you want, provide the boiler serial number, how long the fault has been present, any recent work or power outages, and photos of the display and inside the casing and I can suggest the most likely next step to give to your engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston S30 System Gas Boiler.