Ariston Egis 24 MFFI Fault codes & diagnostics

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

Combi Natural Gas 24 kW 2005-2010 Discontinued

Replaced by: Egis Plus

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.
7 Emergency 40 High 3 Medium 9 Low
Just want the most common Ariston Egis 24 MFFI faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 59 documented codes

Tap any card for details

05

Your boiler has detected very cold temperatures and is running the pump automatically to prevent the internal pipes from freezing.

Low DIY-safe

What to check first

  1. Ensure the boiler remains plugged in and the mains power switch is kept on
  2. Check that your external condensate pipe is not frozen if temperatures are below zero
  3. Increase the room thermostat slightly to help warm the system pipes naturally

Related codes

Full explanation for 05
06

Your boiler has detected very cold temperatures and is running a built-in safety cycle to prevent the internal pipes from freezing.

Low DIY-safe

What to check first

  1. Ensure the gas supply is turned on
  2. Check that the boiler power is switched on so the pump can circulate water
  3. Increase the room thermostat temperature slightly to warm the property
  4. Check for any frozen external pipes (condensate pipe) and defrost with warm water if necessary

Related codes

Full explanation for 06
A01

Your boiler has switched itself off to prevent damage because its internal components have become too hot.

High DIY-safe £100-£280

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure your system water pressure is between 1 and 1.5 bar
  3. Press and hold the Reset button for 5 seconds to clear the lockout

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • NTC Temperature Sensor

Related codes

Full explanation for A01
A03

Your boiler has failed to light after several attempts and has locked itself out for safety.

High DIY-safe £100-350

What to check first

  1. Check that your gas emergency control valve is open and you have credit on your gas meter
  2. Check if other gas appliances in your home, such as a hob, are working correctly
  3. Press the Reset button on the boiler control panel to attempt a restart

Parts commonly replaced

  • Ignition electrode
  • Flame sensing electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for A03
A36

Your boiler has detected that the exhaust gases are getting too hot, which has caused it to shut down for safety.

Emergency Engineer only £150-450

Parts commonly replaced

  • Main Heat Exchanger
  • Flue NTC Sensor
  • Burner Assembly

Related codes

Full explanation for A36
A37

Your boiler's internal computer has encountered a critical error and can no longer process instructions to run the system.

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for A37
A39

Your boiler's internal computer has developed an electronic error and can no longer control the system safely.

High Engineer only £250-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for A39
A40

Your boiler's internal computer brain is reporting a critical communication error and cannot operate safely.

High Engineer only £250-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for A40
A41

Your boiler has stopped working because the fan that removes waste gases cannot spin at the correct speed or has failed to start.

High Engineer only £180-350

Parts commonly replaced

  • Fan motor
  • Air pressure switch
  • Main PCB

Related codes

Full explanation for A41
AIR PRESSURE SWITCH N.O.

Your boiler has stopped working because it cannot safely confirm that the fan is moving air and fumes through the system correctly.

High Engineer only £120-280

Parts commonly replaced

  • Air Pressure Switch
  • Fan
  • Venturi Tube
Full explanation for AIR PRESSURE SWITCH N.O.
Showing 11–20 of 59
APPLIANCE COMPLETELY SHUT DOWN

Your boiler has lost all power or cannot start up at all, leaving you without any heating or hot water.

High DIY-safe £100-350

What to check first

  1. Check if your home's main fuse box has tripped any switches.
  2. Ensure the fused spur switch next to the boiler is turned ON.
  3. Check if your room thermostat is set high enough to call for heat.
  4. Check if your prepaid gas or electric meter has credit.

Parts commonly replaced

  • Internal Fuse
  • Circulating Pump
  • Overheat Thermostat
  • Printed Circuit Board (PCB)
Full explanation for APPLIANCE COMPLETELY SHUT DOWN
AUTODIAGNOSIS FAULTY

Your boiler's internal computer has detected a general processing error and cannot tell exactly what is wrong, causing the system to stop working for safety.

High Engineer only £180-350

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Loom

Related codes

Full explanation for AUTODIAGNOSIS FAULTY
Autodiagnostics faulty

Your boiler's computer system has detected an internal error, meaning it can no longer safely monitor or control the heating process.

High Engineer only £180-350

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for Autodiagnostics faulty
E02

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

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath the boiler.
  2. Slowly open the valve(s) to allow water into the system while watching the pressure gauge.
  3. Close the valve(s) tightly once the pressure reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel
Full explanation for E02
E04

Your boiler is failing to light the gas burner after several attempts, meaning you will not have any heating or hot water until the issue is resolved.

High DIY-safe £100-350

What to check first

  1. Check that your gas emergency control valve is in the 'on' position
  2. If you have a prepay gas meter, check that you have sufficient credit
  3. Try to reset the boiler by pressing the reset button for 5 seconds
  4. Check if other gas appliances in your home, like a hob, are working correctly

Parts commonly replaced

  • Ignition electrode
  • Flame sensing electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E04
E05

Your boiler has sensed that the temperature inside is very low and has automatically turned on the internal pump to prevent the water from freezing.

Low DIY-safe

What to check first

  1. Ensure your electricity supply to the boiler remains switched on.
  2. Verify that your external condensate pipe is not frozen or blocked.
  3. Open any radiator valves in cold rooms to help the water circulate.
  4. Check that your main gas supply is active so the burner can fire if the temperature drops further.

Related codes

Full explanation for E05
E06

Your boiler has detected freezing or extremely cold temperatures inside the system and is attempting to prevent damage by running a safety cycle.

Medium DIY-safe £0-120

What to check first

  1. Ensure your household heating is turned on to help thaw any frozen internal components.
  2. Check that the room where the boiler is located is adequately heated.
  3. Restart the boiler once the surrounding area has warmed up.

Parts commonly replaced

  • NTC Temperature Sensor
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E06
E07

Your boiler has stopped working because water is not moving through the system properly to keep the internal parts cool.

High DIY-safe £150-450

What to check first

  1. Check that your boiler pressure gauge is between 1 and 1.5 bar
  2. Open all radiator valves fully to ensure water can flow
  3. Bleed your radiators to remove any trapped air pockets

Parts commonly replaced

  • Circulation pump
  • Primary heat exchanger
  • Water pressure sensor

Related codes

Full explanation for E07
E08

Your boiler has stopped working because the water pressure is too low for it to operate safely.

High DIY-safe £60-£120

What to check first

  1. Locate the filling loop (the flexible silver hose) underneath the boiler.
  2. Slowly open the one or two taps on the loop until you hear water moving.
  3. Watch the pressure gauge and close the taps once it reaches between 1 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for E08
E11

Your boiler has lost its connection to the sensor that monitors water temperature, meaning it cannot safely heat your water or radiators.

High Engineer only £90-180

Parts commonly replaced

  • Flow NTC Temperature Sensor
  • Wiring harness
  • Main PCB

Related codes

Full explanation for E11
Showing 21–30 of 59
E12

Your boiler has stopped working because one of its internal sensors is sending an incorrect electrical signal to the main control board.

High Engineer only £120-£200

Parts commonly replaced

  • Flow temperature NTC thermistor
  • Wiring harness
  • Main PCB

Related codes

Full explanation for E12
E13

Your boiler has lost connection with the sensor that monitors the temperature of the water returning from your radiators.

High Engineer only £90-160

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness connection

Related codes

Full explanation for E13
E14

Your boiler has detected a electrical fault with the internal sensor that monitors the temperature of the water returning to the unit, causing it to stop heating for safety.

High Engineer only £90-160

Parts commonly replaced

  • Return NTC Temperature Sensor
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for E14
E15

Your boiler has lost its connection to the outside temperature sensor, which helps it run more efficiently based on the weather.

Low Engineer only £70-130

Parts commonly replaced

  • Outdoor temperature sensor
  • NTC sensor wiring

Related codes

Full explanation for E15
E16

Your boiler's outside temperature sensor has developed a wiring fault, causing it to lose track of the weather conditions.

Low Engineer only £90-160

Parts commonly replaced

  • Outdoor temperature sensor
  • Sensor wiring

Related codes

Full explanation for E16
E18

Your boiler is struggling to keep the flame stable and attached to the burner, which prevents it from heating properly.

High Engineer only £120-350

Parts commonly replaced

  • Burner
  • Gas Valve
  • Flame Sensing Electrode
  • Fan

Related codes

Full explanation for E18
E20

Your boiler thinks a flame is still burning even though the gas supply has been shut off, which is a safety conflict.

Emergency Engineer only £120-350

Parts commonly replaced

  • Ionisation Probe
  • Gas Valve
  • Printed Circuit Board (PCB)
Full explanation for E20
E22

Your boiler has stopped working because it cannot confirm that waste gases are being safely ventilated through the flue pipe.

High Engineer only £120-190

Parts commonly replaced

  • Air Pressure Switch
  • Venturi Tube
  • Fan

Related codes

Full explanation for E22
E23

Your boiler has stopped working because the internal fan isn't spinning fast enough to safely clear waste gases from the system.

High Engineer only £180-350

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E23
E25

Your boiler has lost its connection to the temperature sensor that monitors the water in your tank, preventing it from heating your water safely.

High Engineer only £90-180

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for E25
Showing 31–40 of 59
E26

Your boiler has detected an electrical fault with the sensor that monitors the temperature of your hot water tank, meaning it cannot heat water safely.

High Engineer only £100-£180

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for E26
E27

Your boiler has detected an issue with the internal water tank or its temperature sensor, which may limit your access to hot water.

Medium Engineer only £100-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Storage Tank Thermostat
  • Internal Wiring Harness

Related codes

Full explanation for E27
E29

Your boiler has lost its connection to the room thermostat or internal temperature sensor, meaning it doesn't know when to turn the heating on or off.

Medium DIY-safe £80-180

What to check first

  1. Check if your room thermostat or programmer has run out of batteries and replace them if necessary.
  2. Ensure the thermostat is turned up higher than the current room temperature.
  3. Reset the boiler by turning the mode selector dial to 'Reset' for five seconds and then back to its original position.

Parts commonly replaced

  • Room thermostat batteries
  • External room thermostat
  • Internal wiring loom
  • Temperature sensor (NTC probe)

Related codes

Full explanation for E29
E30

Your boiler has lost its connection to the internal room temperature sensor, meaning it cannot tell how warm the house is and has stopped running for safety.

High Engineer only £90-180

Parts commonly replaced

  • Room Temperature Sensor (NTC)
  • Wiring Harness
  • Main Control Board

Related codes

Full explanation for E30
E33

Your boiler has stopped running because it has detected a safety issue with the floor heating temperature sensor or its wiring.

High Engineer only £90-180

Parts commonly replaced

  • Clamp-on thermostat
  • Floor temperature sensor
  • Wiring harness

Related codes

Full explanation for E33
E34

Your boiler has detected a communication problem with the safety sensor that monitors the exhaust gases, causing the system to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • Flue NTC sensor
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for E34
E35

Your boiler has detected an electrical fault with the sensor that monitors exhaust gases, causing the system to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • Flue gas sensor (NTC)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for E35
E36

Your boiler has shut down because the exhaust fumes have reached a dangerously high temperature, indicating a potential blockage or serious heat exchange issue.

Emergency Engineer only £150-£450

Parts commonly replaced

  • Main Heat Exchanger
  • Flue NTC Sensor
  • Flue Ducting

Related codes

Full explanation for E36
E37

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

High Engineer only £250-£450

Parts commonly replaced

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

Related codes

Full explanation for E37
E38

Your boiler has locked itself out because the reset button was pressed too many times in a short period.

Low DIY-safe £0-80

What to check first

  1. Turn the boiler power off at the fused spur switch (the wall switch).
  2. Wait for at least 5 to 10 minutes to allow the internal memory to clear.
  3. Turn the power back on and wait for the boiler to initialize without pressing any buttons.

Parts commonly replaced

  • None (usually a software lockout)
  • PCB (if the button is physically stuck)
Full explanation for E38
Showing 41–50 of 59
FLAME DETECTED

Your boiler thinks it sees a flame even when the gas should be turned off, which prevents it from starting up safely.

High Engineer only £120-350

Parts commonly replaced

  • Flame sensing electrode
  • Ionisation probe
  • Main PCB
Full explanation for FLAME DETECTED
flame failure reset

Your boiler is failing to light the internal flame needed to heat your water, so it has shut down as a safety precaution.

High DIY-safe £120-250

What to check first

  1. Check that your gas supply is turned on and other gas appliances are working.
  2. Locate the reset button or turn the control knob to the central 'reset' position.
  3. Hold the reset button for 3 seconds and wait for the boiler to attempt an ignition cycle.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB board

Related codes

Full explanation for flame failure reset
I / Fume Sensor

Your boiler has detected an issue with the system that ensures dangerous exhaust fumes are safely removed from your home, so it has shut itself down as a safety precaution.

Emergency Engineer only £120-250

Parts commonly replaced

  • Flue safety thermostat
  • PCB
  • Flue fan assembly
Full explanation for I / Fume Sensor
Ignition failure

Your boiler has stopped working because it cannot light the flame required to produce heat or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Ignition failure
Insufficient System Pressure

Your boiler has switched itself off because the water level inside your central heating system is currently too low for it to operate safely.

High DIY-safe £80-150

What to check first

  1. Locate the external filling loop (a flexible metal braided hose) underneath the boiler.
  2. Open the small valve(s) on the loop to allow mains water into the heating system.
  3. Monitor the pressure gauge on the boiler front until the needle reaches 1.5 bar.
  4. Close the valve(s) tightly and ensure no water is dripping from the loop connection.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure gauge
Full explanation for Insufficient System Pressure
L.E.D. E (Yellow)

Your boiler has detected a problem with clearing its exhaust fumes and has safely turned itself off to prevent carbon monoxide buildup.

Emergency Engineer only £120-250

Parts commonly replaced

  • Air Pressure Switch
  • Ventilation Fan
  • Venturi
  • Flue Seal

Related codes

Full explanation for L.E.D. E (Yellow)
L.E.D. I

Your boiler is failing to light the internal flame, which means it cannot provide you with heating or hot water until the issue is fixed.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition transformer
  • Printed Circuit Board (PCB)

Related codes

Full explanation for L.E.D. I
L.E.D. I (Red)

Your boiler has automatically turned itself off because the internal water temperature has become dangerously high.

High Engineer only £150-300

Parts commonly replaced

  • NTC thermistor sensor
  • Pump
  • Primary heat exchanger
  • PCB

Related codes

Full explanation for L.E.D. I (Red)
MAIN FLOW SWITCH CLOSED

Your boiler thinks water is moving through the pipes when the pump is actually off, which is preventing it from starting up safely.

High Engineer only £120-220

Parts commonly replaced

  • Main Flow Switch
  • Water Pressure Switch
  • Circulation Pump

Related codes

Full explanation for MAIN FLOW SWITCH CLOSED
Pressure gauge D < 0.5 bar

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

High DIY-safe £0-80

What to check first

  1. Locate the filling loop, which is usually a flexible silver braided hose with one or two valves underneath the boiler.
  2. Open the valve(s) slowly to allow mains water into the system while watching the pressure gauge on the front of the boiler.
  3. Once the needle reaches the green zone (between 1.0 and 1.5 bar), tightly close the valve(s) to stop the flow.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel
Full explanation for Pressure gauge D < 0.5 bar
Showing 51–59 of 59
PUMP FED

Your boiler's internal pump is not moving water through the system correctly, which is preventing the unit from heating up safely.

High Engineer only £180-£350

Parts commonly replaced

  • Circulation Pump
  • Pump Capacitor
  • Main PCB

Related codes

Full explanation for PUMP FED
Red L.E.D. I illuminated

Your boiler has failed to start the flame after several attempts and has shut itself down as a safety precaution.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board

Related codes

Full explanation for Red L.E.D. I illuminated
Red LED I

Your boiler has stopped working because it failed to light the flame needed to heat the water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Red LED I
Red LED I (Overheat)

Your boiler has automatically turned itself off because it has detected that the water inside is getting dangerously hot.

High Engineer only £120-250

Parts commonly replaced

  • NTC sensor
  • Primary heat exchanger
  • Pump

Related codes

Full explanation for Red LED I (Overheat)
TCS STATE

Your boiler is currently in a normal waiting state because it has reached the required temperature and is managing its own internal heating cycle.

Low DIY-safe

What to check first

  1. Check that your room thermostat or programmer is set to a high enough temperature to call for heat
  2. Wait 5 to 10 minutes for the boiler to complete its anti-cycling period
  3. Ensure your radiator valves are open if you are testing the central heating
Full explanation for TCS STATE
TCS STATE (EXTERNAL)

Your boiler is currently on standby because your thermostat is set too low or the boiler is switched to summer mode, meaning it is waiting for a request for heat.

Low DIY-safe £0-60

What to check first

  1. Check if the boiler is set to 'Summer' mode (hot water only) and switch it to 'Winter' mode if you need heating.
  2. Turn up your room thermostat or programmer to ensure it is calling for heat.
  3. Check if the batteries in your wireless room thermostat have run out and replace them if necessary.

Parts commonly replaced

  • External Thermostat Batteries
  • Room Thermostat
  • Programmer
Full explanation for TCS STATE (EXTERNAL)
WATER AT REQUIRED TEMPERATURE

Your boiler has reached its target heat and is temporarily pausing circulation to prevent overheating.

Low DIY-safe

What to check first

  1. Check that your room thermostat is turned up
  2. Ensure radiator valves are open to allow heat to dissipate
  3. Monitor the boiler to see if it fires up again once the temperature drops
Full explanation for WATER AT REQUIRED TEMPERATURE
Yellow L.E.D. H

Your boiler has detected that fumes are not being properly vented outside, so it has safely shut itself down to prevent a carbon monoxide hazard.

Emergency Engineer only £120-250

Parts commonly replaced

  • Flue fan
  • Air pressure switch
  • Venturi tube
Full explanation for Yellow L.E.D. H
Yellow L.E.D. H illuminated

Your boiler has temporarily turned itself off because it is struggling to safely clear away waste gases, which could lead to harmful fumes building up if not addressed.

Emergency Engineer only £120-250

Parts commonly replaced

  • Flue fan
  • Air pressure switch
  • Venturi
  • Flue seals
Full explanation for Yellow L.E.D. H illuminated