Common problems guide

The most common Keston C55 problems

The faults most likely to send a Keston C55 into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
26 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
E 44 High DIY-safe

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

What to try first

Locate the external filling loop, usually two silver valves connected by a braided metal hose underneath the boiler.Gradually open the valves to let mains water into the system until the pressure gauge on the front reads between 1.0 and 1.5 bar.Close both valves firmly and check for any visible leaks around your radiators or pipework.

Est. cost
£80-150
Parts
Pressure relief valve, Pressure sensor, Expansion vessel
Full guide for E 44
02
B 26 High DIY-safe

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

What to try first

Locate the external filling loop, which is usually a flexible silver braided hose connecting two pipes near the boiler.Open the valves at both ends of the hose to allow water into the system while watching the pressure gauge on the front of the boiler.Stop filling when the needle reaches the green zone (typically between 1.0 and 1.5 bar) and ensure both valves are tightly closed.

Est. cost
£80-150
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for B 26
03
E 02 High Engineer

Your boiler is failing to light because it cannot detect a flame, meaning you currently have no heating or hot water.

Est. cost
£150-350
Parts
Ignition electrode, Gas valve, Spark generator
Full guide for E 02
04
E 04 High Engineer

Your boiler has detected that the power was turned off and back on again while it was already trying to deal with a fault, causing the system to lock itself for safety.

Est. cost
£120-250
Parts
Ignition electrode, Gas valve, PCB
Full guide for E 04
05
E 11 High Engineer

Your boiler's main computer brain is experiencing an internal error and cannot operate safely, meaning you currently have no heating or hot water.

Est. cost
£300-500
Parts
Main Control Board, PCB
Full guide for E 11
06
E 18 High Engineer

Your boiler has stopped working because it has detected that the water inside is significantly hotter than it should be.

Est. cost
£150-300
Parts
Central heating pump, System filter, Flow and return thermistor
Full guide for E 18
Parts most often replaced

on the Keston C55

Pressure relief valvePressure sensorExpansion vesselFilling loopIgnition electrodeGas valve

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Keston shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 26-code list for the Keston C55

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →