Common problems guide

The most common Saunier Duval Thelia 23E problems

The faults most likely to send a Saunier Duval Thelia 23E 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.
33 documented codes
6 most common
3 DIY-safe checks
3 engineer needed

The 6 most common faults

01
E21 High DIY-safe

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

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 mains water into the system until the pressure gauge reads between 1.0 and 1.5 bar.Close the valve(s) tightly and check the boiler for any signs of leaks.

Est. cost
£0-£100
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for E21
02
0.5bar 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 hose connected to two valves beneath the boiler.Open both valves slowly to allow cold mains water to enter the system until the pressure gauge on the front reads between 1.0 and 1.5 bar.Ensure both valves are fully closed tightly and check the boiler display to see if the error has cleared.

Parts
Filling loop, Pressure relief valve, Expansion vessel
Full guide for 0.5bar
03
2.7bar High DIY-safe

Your boiler's internal water pressure is currently too high, which can cause the safety valve to leak or performance issues.

What to try first

Locate a radiator in your home with a bleed valve.Place a cloth and a container underneath the valve.Open the bleed valve slightly to release a small amount of water until the pressure gauge on the front of the boiler drops to between 1.0 and 1.5 bar.

Est. cost
£80-150
Parts
Pressure relief valve, Pressure gauge, Filling loop
Full guide for 2.7bar
04
E01 High Engineer

Your boiler is trying to start but cannot detect a flame, so it has safely shut down to prevent gas from building up.

Est. cost
£120-250
Parts
Ignition electrode, Gas valve, PCB
Full guide for E01
05
E02 High Engineer

Your boiler is struggling to clear exhaust gases properly, so it has safely shut itself down to prevent any unsafe buildup.

Est. cost
£120-250
Parts
Air pressure switch, Flue fan, Venturi tube
Full guide for E02
06
E03 High Engineer

Your boiler is struggling to clear exhaust fumes safely, so it has purposefully shut down to prevent any risk to your home.

Est. cost
£120-250
Parts
Air pressure switch, Flue fan, Venture tube or silicone tubing
Full guide for E03
Parts most often replaced

on the Saunier Duval Thelia 23E

Filling loopPressure relief valvePressure sensorExpansion vesselPressure gaugeIgnition electrode

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 Saunier Duval 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 33-code list for the Saunier Duval Thelia 23E

Related guides

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