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.

The 6 most common faults

01

01 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: £120-250
Parts: Ignition electrode, Gas valve, Ignition transformer

Full guide for 01

02

06 High Engineer

Your boiler has lost the ability to accurately measure the temperature of the water used for your radiators, causing it to stop heating as a safety precaution.

Est. cost: £120-220
Parts: NTC Temperature Sensor, Wiring harness

Full guide for 06

03

12 High Engineer

Your boiler is having trouble communicating with its control panel, which prevents it from receiving accurate instructions to heat your home.

Est. cost: £120-250
Parts: User interface display board, Communication cable/wiring harness, Main PCB

Full guide for 12

04

13 High Engineer

Your boiler's central control unit is experiencing an internal electronic failure and requires professional intervention to restore operation.

Est. cost: £250-450
Parts: Main printed circuit board (PCB)

Full guide for 13

05

14 High Engineer

Your boiler has detected that the water inside the heating system has become dangerously hot and has shut itself down to prevent damage.

Est. cost: £120-250
Parts: NTC Temperature Sensor, Central Heating Pump, Primary Heat Exchanger

Full guide for 14

06

15 High Engineer

Your boiler is struggling to control its gas supply, which prevents it from safely igniting or maintaining a flame.

Est. cost: £180-350
Parts: Gas valve, PCB

Full guide for 15

Parts most often replaced

on the Saunier Duval Thelia

Ignition electrodeGas valveIgnition transformerNTC Temperature SensorWiring harnessUser interface display board

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 12-code list for the Saunier Duval Thelia