How much does it cost to fix Keston error code B 18?

Typical repair cost for the Keston B 18 fault is £150-350 including labour.

B 18 High

Keston B 18 Fault Code

Q: What does the Keston B 18 fault code mean?

Your boiler has detected that the water inside the system is dangerously hot and has shut itself down to prevent damage or potential overheating.

Also written as B18 — same fault.

The Keston B 18 fault code means: your boiler has detected that the water inside the system is dangerously hot and has shut itself down to prevent damage or potential overheating.. It appears on 3 Keston models in our database.

Technical description: Flow temperature exceed 95oC

Typical repair cost £150-350 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Keston models that show the B 18 fault code

Click a model for the full fault code list and model-specific repair notes.

C40 Repair: £150-300
C55 Repair: £150-350
C90 / C110 Repair: £150-350

Source

Keston B 18 fault code information on this page is sourced and cross-checked against the official Keston service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026