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.
20 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
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
02
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
03
01 High Engineer

Your boiler has failed to light because it is not receiving gas or cannot sense the flame, so it has safely shut itself down to prevent a gas buildup.

Est. cost
£120-250
Parts
Ignition electrode, Gas valve, Ignition lead
Full guide for 01
04
02 High Engineer

Your boiler is not detecting that the air extraction system is working correctly, so it has safely shut down to prevent unsafe operation.

Est. cost
£150-250
Parts
Air pressure switch, Flue fan unit, PCB
Full guide for 02
05
05 High Engineer

Your boiler has detected that it is running too hot and has automatically shut down to prevent damage to its internal parts.

Est. cost
£120-250
Parts
NTC temperature sensor, Pump, Heat exchanger
Full guide for 05
06
06 High Engineer

Your boiler has lost the ability to measure the temperature of the water it is heating, so it has stopped working as a safety precaution.

Est. cost
£120-220
Parts
NTC temperature sensor, Wiring harness
Full guide for 06
Parts most often replaced

on the Saunier Duval Thelia 23E

Filling loopPressure relief valveExpansion vesselPressure gaugeIgnition 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 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 20-code list for the Saunier Duval Thelia 23E

Related guides

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