Common problems guide

The most common Keston C40 problems

The faults most likely to send a Keston C40 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
3 DIY-safe checks
3 engineer needed

The 6 most common faults

01
B 26 High DIY-safe

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

What to try first

Locate the filling loop (usually a silver braided hose with one or two valves) underneath the boiler.Slowly open the valve(s) to allow cold mains water into the system while watching the pressure gauge.Stop when the needle reaches the green zone, typically between 1.0 and 1.5 bar, then ensure valves are fully closed.

Est. cost
£80-150
Parts
Pressure Relief Valve (PRV), Expansion Vessel, Filling Loop
Full guide for B 26
02
E 03 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 filling loop, which is a silver braided hose with one or two valves attached under the boiler.Gradually open the valve(s) to allow cold mains water into the system while watching the boiler's pressure gauge.Close the valve(s) tightly once the needle reaches the green zone, usually between 1.0 and 1.5 bar.

Est. cost
£80-150
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for E 03
03
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, which is a silver braided flexible hose connected between two valves underneath the boiler.Open the valves slowly to allow mains water to enter the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.Close both valves tightly to prevent over-pressurisation and check the boiler display to see if the fault has cleared.

Est. cost
£80-150
Parts
Pressure relief valve, Filling loop, Pressure sensor
Full guide for E 44
04
bxx High Engineer

Your boiler has detected that the water inside the system has become dangerously hot or the water pressure is too low, causing it to shut down to prevent internal damage.

Est. cost
£150-350
Parts
Pressure relief valve, Central heating pump, Primary flow sensor
Full guide for bxx
05
E 02 High Engineer

Your boiler has attempted to start but failed to light the flame, so it has safely shut itself down to prevent a build-up of unburnt gas.

Est. cost
£150-300
Parts
Ignition electrode, Gas valve, Ignition lead
Full guide for E 02
06
B 19 High Engineer

Your boiler has detected that the water returning to it is dangerously hot, causing it to shut down to prevent internal damage.

Est. cost
£150-300
Parts
NTC temperature sensor, Central heating pump, Heat exchanger
Full guide for B 19
Parts most often replaced

on the Keston C40

Pressure Relief Valve (PRV)Expansion VesselFilling LoopFilling loopPressure relief valvePressure sensor

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 C40

Related guides

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