Saunier Duval Thelia Fault codes & diagnostics

19 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 23-30 kW 1995-2005 Discontinued

Replaced by: Thelia Condens

Download the Saunier Duval Thelia manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 16 High 1 Medium 1 Unrated
Just want the most common Saunier Duval Thelia faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 19 documented codes

Tap any card for details

0.5bar

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

High DIY-safe £0 - £120

What to check first

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

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel
Full explanation for 0.5bar
01

Your boiler is failing to light because it cannot detect a flame, meaning you currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition transformer

Related codes

Full explanation for 01
02

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

High Engineer only £150-300

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Printed circuit board (PCB)
Full explanation for 02
2.7bar

Your boiler's internal water pressure is currently too high, which can put unnecessary strain on the system's internal components.

Medium DIY-safe £80-150

What to check first

  1. Locate a radiator in your home that is easy to access.
  2. Open the radiator bleed valve slowly with a key until water begins to drip out.
  3. Allow a small amount of water to drain into a bowl until the boiler's pressure gauge drops back into the green zone (usually 1.0 to 1.5 bar), then tightly close the valve.

Parts commonly replaced

  • Pressure Relief Valve (PRV)
  • Expansion Vessel
  • Pressure Sensor
Full explanation for 2.7bar
05

Your boiler has detected that it is getting too hot and has automatically switched itself off to prevent any damage or danger.

High Engineer only £120-250

Parts commonly replaced

  • NTC thermistor
  • Primary heat exchanger
  • Pump
  • Overheat thermostat
Full explanation for 05
06

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.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness
Full explanation for 06
09

Your boiler is struggling to accurately detect the water pressure inside the system, which is preventing it from safely starting up.

High Engineer only £120-220

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness
Full explanation for 09
12

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

High Engineer only £120-250

Parts commonly replaced

  • User interface display board
  • Communication cable/wiring harness
  • Main PCB
Full explanation for 12
13

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

High Engineer only £250-450

Parts commonly replaced

  • Main printed circuit board (PCB)
Full explanation for 13
14

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

High Engineer only £120-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Central Heating Pump
  • Primary Heat Exchanger

Related codes

Full explanation for 14
15

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

High Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • PCB
Full explanation for 15
16

Your boiler has detected a problem with the internal mechanism that controls the gas supply, meaning it has shut down to keep your home safe.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for 16
17

Your boiler is failing to start because the electrical supply it is receiving from your home's wiring is too weak or unstable.

High Engineer only £120-250

Parts commonly replaced

  • Printed circuit board
  • Wiring harness
Full explanation for 17
18

Your boiler's electronic display or control panel is having trouble communicating with the main system, causing the unit to stop operating.

High Engineer only £150-300

Parts commonly replaced

  • User interface board
  • Display cable harness
  • Main printed circuit board
Full explanation for 18
19

Your boiler has lost its connection to the temperature sensor that monitors your heating water, so it has safely shut down to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness
Full explanation for 19
20

Your boiler's electronic control panel is not communicating correctly with the main internal system, preventing it from functioning.

High Engineer only £150-300

Parts commonly replaced

  • User Interface (UI) Board
  • Main Printed Circuit Board (PCB)
  • Communication cable
Full explanation for 20
F1

Your boiler is attempting to start its flame but is failing to maintain it, which means you currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Ionisation probe
  • Gas valve
  • PCB

Related codes

Full explanation for F1
F4

Your boiler is attempting to start its flame but is immediately shutting down because it cannot confirm that the fire has actually caught.

High Engineer only £120-250

Parts commonly replaced

  • Flame rectification probe
  • Ignition lead
  • Printed Circuit Board (PCB)
Full explanation for F4