Saunier Duval Isotwin F30E Fault codes & diagnostics

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

Combi Natural Gas 30 kW 2003-2010 Discontinued

Replaced by: Isotwin Condens

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Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 28 High
Just want the most common Saunier Duval Isotwin F30E faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 29 documented codes

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21

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

High DIY-safe

What to check first

  1. Locate the blue filling loop tap positioned underneath the boiler casing.
  2. Slowly turn the tap to allow cold mains water into the system until the pressure gauge reads 1.5 bar.
  3. Close the tap firmly and check the boiler display to see if the error has cleared.

Parts commonly replaced

  • Pressure relief valve
  • Pressure sensor
  • Expansion vessel
Full explanation for 21
23

Your boiler is struggling to detect water properly circulating through its system, which means it will stop producing heat and hot water as a safety precaution.

High Engineer only £150-350

Parts commonly replaced

  • Diverter valve
  • Flow sensor
  • System pump

Related codes

Full explanation for 23
F1

Your boiler is unable to safely light the flame needed to provide heating and hot water.

High DIY-safe £100-£350

What to check first

  1. Check that your gas prepayment meter has credit (if applicable)
  2. Verify that other gas appliances, such as a hob, are working correctly
  3. Press and hold the reset button (flame symbol with a cross) for five seconds

Parts commonly replaced

  • Ignition electrode
  • Flame sensing electrode
  • Gas valve
  • Ignition PCB

Related codes

Full explanation for F1
F1 or F4

Your boiler is failing to light the flame needed to produce heat and hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for F1 or F4
F1, F4

Your boiler is trying to start but the flame is failing to ignite, meaning you currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Ignition lead
  • Gas valve
  • Printed circuit board (PCB)

Related codes

Full explanation for F1, F4
F2

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

High Engineer only £150-350

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi

Related codes

Full explanation for F2
F4

Your boiler is unable to safely light the flame needed to provide heating and hot water.

High DIY-safe £100-£350

What to check first

  1. Check that your gas prepayment meter has credit (if applicable)
  2. Verify that other gas appliances, such as a hob, are working correctly
  3. Press and hold the reset button (flame symbol with a cross) for five seconds

Parts commonly replaced

  • Ignition electrode
  • Flame sensing electrode
  • Gas valve
  • Ignition PCB

Related codes

Full explanation for F4
F5

Your boiler has switched itself off because it has reached an unsafe temperature to protect the internal components from damage.

High DIY-safe £120-300

What to check first

  1. Check that all radiator valves are fully open to allow water to circulate
  2. Ensure the boiler pressure gauge is between 1 and 1.5 bar
  3. Press the reset button once to see if the fault clears after the system has cooled down

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger

Related codes

Full explanation for F5
F6

Your boiler has detected a problem with the sensor that monitors the temperature of the water heading to your radiators, causing the system to shut down for safety.

High Engineer only £100-£180

Parts commonly replaced

  • Central heating flow thermistor (NTC sensor)
  • Connecting wiring harness

Related codes

Full explanation for F6
F7

Your boiler has detected a problem with the sensor that monitors hot water temperature, which typically means you will have little or no hot water.

High Engineer only £100-£190

Parts commonly replaced

  • DHW Thermistor (NTC sensor)
  • Wiring harness / Connector

Related codes

Full explanation for F7
Showing 11–20 of 29
F8

Your boiler's temperature sensor for the hot water storage tank has developed a fault, meaning the system cannot safely monitor or heat your hot water.

High Engineer only £120-£210

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for F8
F9

Your boiler has detected an issue with the sensor that monitors water pressure, which means it cannot verify if there is enough water to run safely.

High DIY-safe £140-230

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1.0 and 1.5 bar.
  2. If the pressure is too low, use the filling loop underneath the boiler to carefully top it up.
  3. Press the reset button (usually marked with a flame or 'reset') once the pressure is corrected.

Parts commonly replaced

  • Water pressure sensor
  • Pressure relief valve
  • Wiring harness

Related codes

Full explanation for F9
F10

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning to the unit, which has caused the heating to stop working.

High Engineer only £100-180

Parts commonly replaced

  • Central heating return thermistor (NTC sensor)
  • Wiring harness connecting the sensor

Related codes

Full explanation for F10
F11

Your boiler is having trouble communicating with its control panel, which means it isn't receiving the instructions it needs to provide heating or hot water.

High Engineer only £120-350

Parts commonly replaced

  • User Interface PCB
  • Main Circuit Board (PCB)
  • Internal Wiring Harness

Related codes

Full explanation for F11
F12

Your boiler's internal computer has lost communication with the control panel or display, preventing the system from operating.

High Engineer only £180-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Display Interface Board
  • Wiring Harness

Related codes

Full explanation for F12
F13

Your boiler's internal computer has lost communication with its electronic components and needs a professional reset or repair.

High Engineer only £180-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for F13
F14

Your boiler has shut down because the water inside the central heating system has reached an unsafely high temperature.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure gauge is between 1.0 and 1.5 bar
  3. Switch the boiler off and back on again to reset the system after it has cooled down

Parts commonly replaced

  • Circulating pump
  • Flow temperature sensor (NTC)
  • Main heat exchanger (flushing/cleaning)

Related codes

Full explanation for F14
F16

Your boiler is still detecting a flame even after it has been told to turn off, which is a safety concern that requires a professional inspection.

Emergency Engineer only £120-350

Parts commonly replaced

  • Gas valve
  • Ionisation probe
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F16
F17

Your boiler has stopped working because the electrical power coming from your home's mains supply is too low for it to run safely.

High Engineer only £100-350

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Low Voltage Transformer

Related codes

Full explanation for F17
F18

Your boiler's control panel and the internal computer are having trouble communicating with each other, meaning the display may not work or respond to your commands.

High Engineer only £180-450

Parts commonly replaced

  • User Interface (Display) PCB
  • Main Control PCB
  • Connection Wiring Harness

Related codes

Full explanation for F18
Showing 21–29 of 29
F19

Your boiler has lost its connection to the sensor that monitors heating temperature, so it has stopped running as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating Flow NTC Sensor
  • Thermistor Wiring Harness

Related codes

Full explanation for F19
F20

Your boiler's internal computer parts are having trouble communicating with each other, causing the system to shut down for safety.

High Engineer only £180-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Display Interface Board
Full explanation for F20
F23

Your boiler has sensed that water is not moving through the system properly, which is causing it to shut down to prevent overheating.

High DIY-safe £150-350

What to check first

  1. Check that the boiler pressure is between 1 and 1.5 bar on the display
  2. Ensure all radiator valves are fully open
  3. Listen for air in the system and bleed radiators if necessary

Parts commonly replaced

  • Circulation pump
  • Temperature thermistors (NTC sensors)
  • Main heat exchanger (de-scaling)

Related codes

Full explanation for F23
F24

Your boiler has stopped working because the water returning from your radiators has become too hot, which usually means the water isn't circulating properly through the system.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves (TRVs) are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Check if any bypass valves in your pipework are accidentally closed

Parts commonly replaced

  • Circulation pump
  • NTC return temperature sensor
  • Main heat exchanger (flushing service)

Related codes

Full explanation for F24
F25

Your boiler has detected that the water temperature is rising too quickly, which usually means the water isn't flowing through the system properly.

High DIY-safe £120-£350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Bleed all radiators to remove trapped air from the system

Parts commonly replaced

  • Circulation pump
  • Main heat exchanger
  • Temperature sensors (NTC thermistors)

Related codes

Full explanation for F25
F26

Your boiler has detected that the water is getting hot too quickly or the temperature difference between pipes is too wide, often caused by poor water flow.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves and isolation valves are fully open
  2. Ensure there is enough water pressure by checking the boiler gauge (target 1.5 bar)
  3. Bleed all radiators to remove trapped air that might be blocking flow
  4. Reset the boiler to see if the fault clears

Parts commonly replaced

  • Circulation pump
  • Main heat exchanger (flushing)
  • Flow/Return thermistor NTC

Related codes

Full explanation for F26
F28

Your boiler has detected that its temperature sensors are giving conflicting readings, which has caused it to stop working for safety.

High Engineer only £120-250

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Main PCB

Related codes

Full explanation for F28
F29

Your boiler has shut down because a safety component has detected that the internal temperature reached a level high enough to potentially damage the system.

High Engineer only £180-350

Parts commonly replaced

  • Main Heat Exchanger
  • Thermal Fuse
  • Wiring Harness

Related codes

Full explanation for F29
Other defects

Your boiler has detected an internal fault that it cannot identify, meaning it has safely shut down to prevent potential damage or unsafe operation.

High Engineer only £150-350

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Sensor assembly

Related codes

Full explanation for Other defects