Common problems guide

The most common Saunier Duval Thelia problems

The faults most likely to send a Saunier Duval Thelia 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
2 DIY-safe checks
4 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 (a silver flexi-hose) underneath the boiler.Open the valves to allow water into the system until the pressure gauge reads between 1.0 and 1.5 bar.Close both valves tightly and restart the boiler.

Est. cost
£0-£120
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for E21
02
0.5bar High DIY-safe

Your boiler has lost pressure, which means there isn't enough water inside the system to circulate heat throughout your home.

What to try first

Locate the filling loop, which is usually a flexible silver hose with one or two valves underneath your boiler.Open the valves slowly to allow mains water into the system while watching the pressure gauge on the front of your boiler.Close the valves firmly once the needle on the gauge reaches between 1.0 and 1.5 bar.

Est. cost
£0 - £120
Parts
Filling loop, Pressure relief valve, Expansion vessel
Full guide for 0.5bar
03
E01 High Engineer

Your boiler is failing to light the pilot flame, which means it cannot produce heating or hot water for your home.

Est. cost
£120-250
Parts
Ignition electrode, Ignition lead, Gas valve
Full guide for E01
04
E02 High Engineer

Your boiler is struggling to clear exhaust fumes safely, so it has stopped working to prevent a hazard.

Est. cost
£120-250
Parts
Air pressure switch, Flue fan, Venture pipe
Full guide for E02
05
E03 High Engineer

Your boiler is currently unable to safely clear away exhaust gases, so it has stopped working to prevent any potential risks.

Est. cost
£120-250
Parts
Air pressure switch, Flue fan, Venturi tube
Full guide for E03
06
E05 High Engineer

Your boiler has detected that it is running too hot and has automatically shut itself down to prevent any damage or danger.

Est. cost
£120-250
Parts
NTC sensor, Overheat thermostat, Pump
Full guide for E05
Parts most often replaced

on the Saunier Duval Thelia

Filling loopPressure relief valvePressure sensorExpansion vesselIgnition electrodeIgnition lead

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

Related guides

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