Keston C2 Fault Code: Causes, Fixes & Repair Costs
What does the Keston C2 fault code mean?
The C2 code on a Keston boiler indicates a BCC fault — a problem with the Boiler Chip Card (BCC). The BCC is a small programmable card that holds your boiler's specific configuration and output settings. When the control board detects that the BCC is missing, mismatched, corrupted, or has lost its stored parameters, it throws the C2 code and shuts the boiler down into block or lockout mode. This is closely related to the C0 code (BCC Start Fault), which appears at start-up; C2 tends to occur once the boiler is already running. Because the BCC is tied directly to the PCB, any recent board work or electronic disturbance is a primary suspect. The boiler will not restart automatically — you must press the reset button once the underlying issue is resolved.
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
- Wrong BCC fitted after a PCB replacement Common
The most common trigger for a C2 fault. When a Primary PCB is replaced on a Keston boiler, a separate Boiler Chip Card specific to that boiler's range and output must also be sourced and fitted. If the engineer used a generic BCC or one coded for a different model, the control board will reject it and display C2. The correct BCC is identified using the serial letter code on the boiler's data plate.
- BCC not properly seated or loose connection Common
The BCC slots into a dedicated socket on the PCB. If the card has worked loose — due to vibration over time or disturbed during a service visit — the board can lose communication with it mid-operation, triggering a C2 lockout. The card must be inserted with the 'TOP' side facing upward as specified in the Keston installation manual.
- BCC data corruption or internal error Sometimes
Like any programmable chip, the BCC can develop an internal fault or suffer data corruption, particularly on older units. The boiler's configuration parameters are lost or unreadable, causing the control board to flag a fault. A reset occasionally clears a transient error, but a genuinely corrupted card will need replacement.
- PCB damaged by a water leak Sometimes
A slow internal leak — for example from the heat exchanger, a pump seal, or a diverter valve — can drip onto the PCB over time, causing board damage that manifests as a BCC fault. If the PCB appears to be at fault but no clear electronic cause is found, an engineer should inspect for underlying water ingress before simply replacing the board.
How to fix it
- Reset the boiler DIY safe
Press and hold the reset button (marked with a spanner symbol on most Keston models) for 3–5 seconds until the display responds. A transient BCC communication glitch can sometimes be cleared this way. Do not reset more than 2–3 times — repeated lockouts without a successful restart indicate a genuine fault.
- Power cycle at the mains DIY safe
Switch the boiler off at the fused spur or isolator switch, wait 30 seconds, then switch it back on. This allows the control electronics to fully discharge and reinitialise, which can resolve minor one-off electronic errors. If the C2 code returns immediately or shortly afterwards, proceed to the next step.
- Check for any obvious water leaks around the boiler DIY safe
Without opening the boiler casing, inspect the area around and beneath the unit for signs of dripping water, staining, or corrosion on external pipework. If you spot a leak, note its location and mention it to the engineer — a leak reaching the PCB is a common root cause that must be fixed before any board replacement.
- Contact a Gas Safe registered engineer with Keston experience Gas Safe engineer
If the fault persists after a reset and power cycle, internal investigation is required. An engineer will need to: inspect the BCC card for physical damage, verify it matches your boiler's model and serial code, check the card is correctly seated, and test the PCB with a multimeter for power and continuity. Keston boilers use a twin-flue modulating design that not every engineer is familiar with — ask specifically whether they have experience with Keston appliances before booking.
- PCB and BCC replacement if diagnosed as faulty Gas Safe engineer
If the board or card is confirmed faulty, the engineer will source a new Primary PCB and the corresponding BCC for your specific boiler variant. The BCC must be oriented correctly (TOP side up) during installation. After fitting, the engineer will carry out a full commissioning check to confirm the boiler locks onto the correct parameters and runs safely.
Parts you may need
- Keston Primary PCB (control board) · from £280
- Boiler Chip Card (BCC) — model-specific · from £55
- PCB mounting harness / connector loom · from £35
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 £150–£500, depending on the underlying cause.
Frequently asked questions
Can I fix a Keston C2 fault myself?
The reset and mains power-cycle steps are safe for a homeowner to try. Beyond that, the C2 fault involves the boiler's PCB and Boiler Chip Card — internal electronic components that must only be handled by a Gas Safe registered engineer. Attempting to remove or reseat the BCC yourself risks further damage and may invalidate any remaining warranty.
Why did I get a C2 code after my boiler was serviced or had its PCB replaced?
This is a very common scenario with Keston boilers. Whenever a PCB is replaced, a separate Boiler Chip Card (BCC) that is specific to your boiler's range and output rating must also be fitted. If the engineer used a card from a different model — or forgot to fit one entirely — the control board will reject it and display C2. The correct BCC is identified using the serial letter code on your boiler's data plate. Ask the engineer to verify this match.
How much does it cost to fix a Keston C2 fault in the UK?
If the fix is simply reseating or replacing the BCC card, costs are typically in the £150–£250 range including a call-out and labour. A full PCB replacement with a new BCC can reach around £450–£500 once parts and labour are accounted for. If the board failure was caused by a water leak, you may also need to repair or replace the leaking component (e.g. a heat exchanger or diverter valve), which adds to the total.
My Keston boiler is over 12 years old and needs a new PCB — is it worth repairing?
It depends on the overall condition of the boiler. If it has been regularly serviced and is otherwise in good shape, a PCB replacement can give it several more years of life. However, if the boiler is 15 years or older, has not been regularly maintained, or if other components are also worn, the cost of a PCB (£450–£500) may make replacement with a modern, more efficient model a wiser investment. A Gas Safe engineer can give you an honest assessment of the boiler's overall condition before you commit to the repair.