C0 Boiler Fault Code: What It Means on Every Brand

The C0 fault code appears on 2 boiler brands — Ideal and Keston — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What C0 means	Severity	DIY?
IIdeal	The C0 code appears on Ideal boilers when the Boiler Chip Card (BCC) has failed to activate correctly. Full guide →	Lockout — engineer likely	No
KKeston	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… Full guide →	Lockout — engineer likely	No

C0 brand by brand

Ideal C0

The C0 code appears on Ideal boilers when the Boiler Chip Card (BCC) has failed to activate correctly. The BCC is a small model-specific card that sits on the PCB and tells the control board exactly which boiler it is running — without a valid BCC signal, the PCB cannot operate the boiler safely and locks it out. A secondary interpretation cited by some sources describes C0 as a central heating signal fault, which can point to circulation or wiring issues rather than the card itself, though the BCC activation failure is the definition most closely aligned with Ideal's own documentation. Either way, the boiler will enter safety lockout and display C0 on its front panel until the underlying problem is resolved.

Causes, fixes & costs for Ideal C0 → · All Ideal codes

Keston C0

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.

Causes, fixes & costs for Keston C0 → · All Keston codes

Frequently asked questions

Is the C0 fault code the same on every boiler?

Broadly, yes — on Ideal and Keston the C0 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a C0 fault myself?

Not usually. On every brand listed, C0 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a C0 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.