Saunier Duval Thelia 23E Fault codes & diagnostics

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

Combi Natural Gas 23 kW Discontinued

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4 Emergency 29 High

All 33 documented codes

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0.5bar

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

High DIY-safe

What to check first

  1. Locate the external filling loop, which is usually a flexible silver hose connected to two valves beneath the boiler.
  2. 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.
  3. Ensure both valves are fully closed tightly and check the boiler display to see if the error has cleared.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel
01

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.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition lead

Related codes

02

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

High Engineer only £150-250

Parts commonly replaced

  • Air pressure switch
  • Flue fan unit
  • PCB

Related codes

2.7bar

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

High DIY-safe £80-150

What to check first

  1. Locate a radiator in your home with a bleed valve.
  2. Place a cloth and a container underneath the valve.
  3. 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.

Parts commonly replaced

  • Pressure relief valve
  • Pressure gauge
  • Filling loop

Related codes

05

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

High Engineer only £120-250

Parts commonly replaced

  • NTC temperature sensor
  • Pump
  • Heat exchanger

Related codes

06

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.

High Engineer only £120-220

Parts commonly replaced

  • NTC temperature sensor
  • Wiring harness

Related codes

09

Your boiler is unable to read how much water is inside the system, so it has safely shut down to prevent damage.

High Engineer only £120-220

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

12

Your boiler is having trouble communicating with the digital control panel, which means it cannot receive instructions to heat your home or water.

High Engineer only £120-220

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Control interface ribbon cable

Related codes

13

Your boiler's central control computer has encountered an internal error and cannot communicate with the system to operate your heating or hot water.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
14

Your boiler has detected that the water inside the unit is getting dangerously hot and has shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • NTC thermistor
  • Primary heat exchanger
  • Central heating pump

Related codes

Showing 11–20 of 33
15

Your boiler is having trouble controlling the flow of gas necessary to ignite the flame, meaning it cannot provide heat or hot water.

High Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Main PCB

Related codes

16

Your boiler has detected an issue with the valve that controls the flow of gas, meaning it cannot safely start up or maintain a flame.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Gas valve solenoid
  • Printed Circuit Board (PCB)
17

Your boiler is receiving an insufficient electrical voltage from your home supply, meaning it lacks the power needed to operate correctly.

High Engineer only £120-250

Parts commonly replaced

  • Printed circuit board
  • Wiring harness
18

Your boiler's control panel is having trouble communicating with the main internal computer, which is preventing the system from operating correctly.

High Engineer only £150-300

Parts commonly replaced

  • User Interface PCB
  • Wiring harness

Related codes

19

Your boiler has lost the connection to a sensor that monitors the water temperature, meaning it has safely shut down to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Central heating NTC temperature sensor
  • Wiring harness

Related codes

20

Your boiler is struggling to communicate with its control panel, meaning it does not recognize the device used to adjust your settings.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • User Interface Module
  • Connection wiring harness
Air flow rate safety device

Your boiler has detected that it cannot safely push exhaust gases outside, so it has shut itself down to prevent a dangerous buildup of fumes in your home.

Emergency Engineer only £120-250

Parts commonly replaced

  • Air pressure switch
  • Flue assembly
  • Fan unit

Related codes

E01

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

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

E02

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

High Engineer only £120-250

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi tube

Related codes

E03

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

High Engineer only £120-250

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venture tube or silicone tubing

Related codes

Showing 21–30 of 33
E05

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

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • NTC thermistor
  • Circulation pump
  • Primary heat exchanger
E06

Your boiler's temperature sensor is providing incorrect readings, which prevents the unit from heating your home safely.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

E09

Your boiler has lost the ability to measure how much water is inside it, so it has safely shut down to prevent any damage.

High Engineer only £150-250

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

E12

Your boiler is struggling to accurately detect the temperature of the water it is heating for your taps, which means it has stopped providing hot water to be safe.

High Engineer only £120-200

Parts commonly replaced

  • DHW NTC Thermistor
  • Wiring harness

Related codes

E13

The main electronic brain of your boiler is experiencing a malfunction and can no longer control the heating or hot water processes correctly.

High Engineer only £250-450

Parts commonly replaced

  • Main printed circuit board (PCB)

Related codes

E14

Your boiler has detected an issue with the system that controls the flow of gas, meaning it cannot safely ignite to provide heating or hot water.

Emergency Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

E15

Your boiler is struggling to accurately control the internal valve that directs heat to your taps or radiators, meaning it cannot reliably provide hot water or heating.

High Engineer only £180-350

Parts commonly replaced

  • Diverter valve motor
  • Diverter valve cartridge
  • Printed circuit board (PCB)
E16

Your boiler is detecting a flame even when it is supposed to be switched off, which prevents it from igniting safely again.

Emergency Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Printed circuit board (PCB)
  • Flame rectification electrode

Related codes

E19

Your boiler has lost its connection to the heat sensor, so it has effectively stopped working because it cannot accurately measure the water temperature.

High Engineer only £120-200

Parts commonly replaced

  • Central heating NTC thermistor
  • Wiring harness
E20

Your boiler has developed a communication error because two of its internal computer chips are not recognising each other correctly, preventing the system from operating.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB
  • Interface PCB

Related codes

Showing 31–33 of 33
E21

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

High DIY-safe £0-£100

What to check first

  1. Locate the filling loop (usually a silver braided hose with one or two valves underneath the boiler).
  2. Slowly open the valve(s) to allow mains water into the system until the pressure gauge reads between 1.0 and 1.5 bar.
  3. Close the valve(s) tightly and check the boiler for any signs of leaks.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor
F1

Your boiler is attempting to start but failing to keep the flame lit, which means it will stop providing heating and hot water for safety reasons.

High Engineer only £120-250

Parts commonly replaced

  • Ionisation electrode
  • Flame sensor
  • Gas valve

Related codes

F4

Your boiler is failing to stay lit because it cannot confirm that the flame has actually started, causing it to shut down for your safety.

High Engineer only £120-250

Parts commonly replaced

  • Flame rectification electrode
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes