Keston C0 Fault Code: Causes, Fixes & Repair Costs
What does the Keston C0 fault code mean?
The C0 fault code on a Keston boiler signals that the PCB (printed circuit board) has been unable to successfully activate or communicate with the BCC — the Boiler Chip Card — during start-up. The BCC is a small, model-specific card that slots onto the PCB and holds all the configuration data unique to that particular boiler's range and output rating. Without a successful handshake between the PCB and BCC at power-on, the boiler locks out and refuses to fire. The closely related C2 code indicates an ongoing BCC fault (rather than a start-up activation failure), so if you see C2 instead, the diagnosis and repair path are very similar to what is described here.
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
- BCC missing or not correctly seated Common
If the BCC was not refitted after a previous PCB replacement, or was dislodged during maintenance, the boiler will immediately throw a C0 at the next start-up. Keston specifies that whenever a replacement PCB is ordered, the correct model-specific BCC must also be purchased separately and installed with the 'TOP' label facing upward. This is a surprisingly common oversight after a PCB swap.
- Failed or corrupted BCC Common
Like any electronic component, the BCC can fail over time — particularly on older boilers. If the card has degraded or its stored configuration data has become corrupted, the PCB cannot read it correctly and locks out with a C0. Replacing the BCC with the correct model-specific card is usually a straightforward fix for a Gas Safe engineer.
- Loose or damaged wiring between BCC and PCB Sometimes
Corrosion, vibration, or accidental damage to the connector or wiring linking the BCC to the PCB can interrupt communication and falsely trigger the C0 fault. An engineer will inspect the connector and surrounding wiring as part of their diagnostic check.
- PCB failure Sometimes
If the PCB itself has failed, it may be incapable of reading the BCC at all, producing a C0 even when the chip card is present and in good condition. PCB failure is more common on boilers over 8–10 years old and typically requires board replacement alongside a new BCC.
How to fix it
- Reset the boiler using the control knob DIY safe
Turn the control mode knob to the reset position, then immediately turn it back to the ON position. The boiler will attempt to fire. This is the manufacturer's recommended first step and takes only a few seconds. Do not repeatedly reset more than two or three times if the fault keeps returning — further resets will not fix an underlying fault and could mask the problem.
- Power-cycle the boiler DIY safe
Switch the boiler off at the mains isolator (or the fused spur), wait a full 30 seconds, then switch it back on. This can clear minor, transient electronic glitches. If the C0 code reappears after power-on, the fault is genuine and persistent — move on to the next step.
- Call a Gas Safe registered engineer with Keston experience Gas Safe engineer
If the reset and power-cycle have not resolved the fault, you need a Gas Safe registered engineer to investigate. Keston boilers have a twin-flue design and a modulating pump that not every engineer is familiar with, so it is worth specifically asking whether they have experience with Keston appliances before booking. The engineer will check whether the BCC is present, correctly oriented, and making good contact with the PCB; test the BCC itself; inspect the PCB wiring and connectors; and replace the BCC or PCB as required. If the boiler is still within its manufacturer's warranty, contact Keston's national service team directly in the first instance.
Parts you may need
- Keston Boiler Chip Card (BCC) — model-specific · from £45
- Keston Primary PCB (replacement board) · from £280
- PCB connector/wiring harness · from £25
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 £100–£350, depending on the underlying cause.
Frequently asked questions
What is the BCC on a Keston boiler and why does it matter?
BCC stands for Boiler Chip Card. It is a small, plastic-housed electronic card that clips onto the PCB and stores all the configuration and calibration data specific to your boiler's model and output rating. Because Keston produces boilers in different output sizes, the BCC is what tells the PCB exactly how to behave. If it is missing, unseated, or faulty, the boiler has no valid configuration to work from and locks out immediately with a C0 (or C2) fault. Whenever a PCB is replaced on a Keston boiler, the correct BCC must be ordered separately and fitted at the same time.
Can I fix a Keston C0 fault myself?
The only safe DIY steps are resetting the boiler via the control knob (up to two or three attempts) and power-cycling it at the mains. Both take less than a minute. If neither clears the fault, the repair involves handling the PCB and BCC — electronic components inside the boiler casing — which must be diagnosed and replaced by a Gas Safe registered engineer. Attempting this yourself would be unsafe and could invalidate any remaining warranty.
How much does it cost to fix a Keston C0 fault in the UK?
If a simple reset or BCC reseating resolves it, you are typically looking at a call-out and labour charge of around £80–£150. A BCC replacement including parts and labour usually falls in the £100–£200 range. If the PCB itself needs replacing, costs rise considerably — typically £400–£650 including parts and labour — and if both the PCB and BCC need replacing together, the total can reach £500–£700 or more. On a boiler that is 12 years old or older, it is worth asking your engineer to weigh the repair cost against a new boiler installation before committing to an expensive PCB replacement.
How long will my Keston boiler be out of action with a C0 fault?
If the engineer can reseat or replace the BCC on the day, you may have heating and hot water back within a couple of hours. If the PCB needs replacing and the part is not in stock, you could be without a working boiler for one to five working days while it is ordered. It is worth asking your engineer to check parts availability before confirming the repair, so you can plan accordingly — especially in cold weather.