Alpha FMK Fault codes & diagnostics

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

Combi Natural Gas Discontinued

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
50 High 6 Medium 1 Low
Just want the most common Alpha FMK faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 57 documented codes

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LED / flash-pattern codes
Flashing red A or B

Your boiler has got too hot and has shut itself down as a safety precaution to prevent damage.

High DIY-safe £100-£250

What to check first

  1. Check that all radiator valves are fully open
  2. Check that the boiler pressure is between 1.0 and 1.5 bar
  3. Turn the selector switch to the 'RESET' position for a few seconds and then back to 'ON'

Parts commonly replaced

  • Overheat Thermostat
  • Temperature Sensor (NTC)
  • Circulation Pump
Full explanation for Flashing red A or B
Digital codes
0A1

Your boiler is unable to light the gas to start the heating process, meaning your home and water will not get warm.

High DIY-safe £100-£350

What to check first

  1. Check that your gas emergency control valve is in the 'open' position
  2. If you have a pre-payment meter, ensure you have sufficient credit
  3. Press and hold the reset button for several seconds to try restarting the boiler

Parts commonly replaced

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

Related codes

Full explanation for 0A1
0A2

Your boiler is struggling to detect a flame, which means it cannot safely stay lit to provide heating or hot water.

High DIY-safe £120-300

What to check first

  1. Check if other gas appliances (like a hob) are working to ensure your gas supply is active
  2. Check that your prepaid gas meter has credit
  3. Hold the reset button for 5 seconds to see if the boiler restarts

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve

Related codes

Full explanation for 0A2
0A3

Your boiler has become too hot and has shut itself down as a safety precaution to prevent damage.

High DIY-safe £100-£350

What to check first

  1. Check that all radiator valves are fully open to allow water to flow.
  2. Ensure there is enough water pressure by checking the gauge is between 1 and 1.5 bar.
  3. Press and hold the reset button for several seconds once the boiler has cooled down.

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Heat Exchanger

Related codes

Full explanation for 0A3
0A5

Your boiler has detected that the fan, which is responsible for clearing exhaust fumes, is not spinning at the correct speed.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 0A5
0A7

Your boiler has sensed that the exhaust gases are getting too hot and has shut down to prevent internal damage.

High Engineer only £120-350

Parts commonly replaced

  • Main Heat Exchanger
  • Flue NTC Sensor
  • Burner Insulation Panel

Related codes

Full explanation for 0A7
0A8

Your boiler is having trouble detecting the flame needed to heat your water, which is causing the system to shut down for safety.

High Engineer only £120-£350

Parts commonly replaced

  • Ionisation probe
  • Flame electrode
  • Ignition lead
  • PCB

Related codes

Full explanation for 0A8
0A9

Your boiler has detected an electrical problem with the component that controls the gas supply, causing it to shut down for safety.

High Engineer only £180-£350

Parts commonly replaced

  • Gas valve
  • Wiring harness
  • Main circuit board (PCB)

Related codes

Full explanation for 0A9
0A15

Your boiler has stopped working because the sensors that monitor water temperature are providing conflicting or incorrect readings.

High Engineer only £120-220

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Wiring harness
Full explanation for 0A15
0A16

Your boiler is struggling to heat the water quickly enough, which usually means there is a blockage or a pump issue preventing water from circulating properly.

High DIY-safe £150-£350

What to check first

  1. Check that your boiler pressure is between 1 and 1.5 bar on the pressure gauge
  2. Ensure all radiator valves are fully open to allow water to flow
  3. Try bleeding your radiators to remove any trapped air

Parts commonly replaced

  • Circulation Pump
  • Main Heat Exchanger
  • Flow Temperature NTC Sensor

Related codes

Full explanation for 0A16
Showing 11–20 of 57
0A17

Your boiler is struggling to heat the water properly because it thinks it is taking too long for the temperature to rise once the burner starts.

High Engineer only £120-280

Parts commonly replaced

  • Return NTC thermistor
  • Pump
  • Heat exchanger
Full explanation for 0A17
0A18

Your boiler has detected an unexpected, rapid temperature change, causing it to shut down for safety while it monitors the water flow.

High DIY-safe £90-180

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. Reset the boiler using the control knob or button

Parts commonly replaced

  • Flow NTC Sensor
  • Return NTC Sensor
  • Circulation Pump

Related codes

Full explanation for 0A18
0A21

Your boiler's main internal computer has developed a serious malfunction, preventing the unit from operating safely.

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Harness
Full explanation for 0A21
0A30

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

High Engineer only £120-£200

Parts commonly replaced

  • NTC Flow Sensor
  • Wiring Harness
Full explanation for 0A30
0A31

Your boiler has detected an unusual temperature reading from an internal sensor and has shut down to prevent damage.

High Engineer only £120-£220

Parts commonly replaced

  • Flow NTC temperature sensor
  • Connecting wiring harness
Full explanation for 0A31
0A37 – Pro Tec 50

Your boiler has detected that water is not circulating through the system quickly enough to prevent overheating, so it has shut down for safety.

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 and 1.5 bar
  3. Check if any external system valves or the lever valves under the boiler are closed

Parts commonly replaced

  • Circulation Pump
  • External System Filter
  • Flow Temperature Sensor
Full explanation for 0A37 – Pro Tec 50
0A43

Your boiler has detected a fault with a temperature sensor that monitors water returning to the unit, causing the system to shut down for safety.

High Engineer only £90-160

Parts commonly replaced

  • Return NTC thermistor
  • Sensor wiring harness
Full explanation for 0A43
0A44

Your boiler has detected that a temperature sensor is sending an incorrect reading, which has caused the system to stop running to prevent damage.

High Engineer only £90-160

Parts commonly replaced

  • Return NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for 0A44
0A80

Your boiler is struggling to monitor the temperature of the water moving through the system, which causes it to shut down after running for a short period.

High Engineer only £120-220

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Wiring harness
Full explanation for 0A80
0E2

Your boiler is struggling to detect a flame, which means it cannot safely stay lit to provide heating and hot water.

High Engineer only £120-350

Parts commonly replaced

  • Ionisation probe
  • Ignition electrode
  • Printed Circuit Board (PCB)
Full explanation for 0E2
Showing 21–30 of 57
0E13

Your boiler has temporarily locked itself because it has been reset too many times in a short period while trying to clear another fault.

High DIY-safe £0-120

What to check first

  1. Turn the power to the boiler off at the fused spur switch
  2. Wait for at least 15 to 30 minutes without touching the reset button
  3. Turn the power back on and see if a different fault code appears

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • PCB
Full explanation for 0E13
0E25

Your boiler's internal computer has developed a fault and is unable to manage the heating and hot water functions correctly.

High Engineer only £250-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring harness connectors
Full explanation for 0E25
0E33

Your boiler has detected that the electricity supply coming into the unit is too low or unstable for it to operate safely.

High DIY-safe £100-350

What to check first

  1. Check if other appliances in your home are flickering or losing power
  2. Check your fuse box (consumer unit) for any tripped switches
  3. If you have a pre-payment meter, ensure you have sufficient credit

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External Voltage Stabiliser
Full explanation for 0E33
0E34

Your boiler is struggling to provide hot water because its internal temperature sensor is giving an impossible reading.

Medium Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC Temperature Sensor
  • Sensor wiring harness

Related codes

Full explanation for 0E34
0E37

Your boiler has stopped working because the water pressure in the central heating system is too low.

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 one or two taps on the loop until you hear water flowing.
  3. Watch the pressure gauge and close the taps when the needle reaches 1.5 bar.
  4. Restart the boiler to clear the fault code.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor
Full explanation for 0E37
0E45

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 £120-210

Parts commonly replaced

  • Flue NTC sensor
  • Wiring harness
Full explanation for 0E45
0E46

Your boiler has detected an unusual temperature reading in the exhaust flue, causing the system to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • Flue NTC Sensor
  • Main PCB
  • Wiring harness
Full explanation for 0E46
0E81

Your boiler has detected that its temperature sensors are giving conflicting or incorrect readings, causing the system to shut down for safety.

High Engineer only £120-220

Parts commonly replaced

  • Flow NTC Temperature Sensor
  • Return NTC Temperature Sensor
  • Wiring Harness
Full explanation for 0E81
0E98

Your boiler has detected that its temperature sensors are giving conflicting or incorrect readings, causing the system to shut down for safety.

High Engineer only £120-210

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Sensor wiring harness
Full explanation for 0E98
0E99

Your boiler is having trouble communicating with other boilers in the system, which may cause it to stop working effectively.

High DIY-safe £120-350

What to check first

  1. Turn the boiler power off at the fused spur switch
  2. Wait for 30 seconds
  3. Turn the power back on and check if the code clears

Parts commonly replaced

  • Cascade communication cable
  • Main PCB
  • Communication interface module
Full explanation for 0E99
Showing 31–40 of 57
12

Your boiler is struggling to read the temperature of the water in your cylinder, which means your hot water system may not function correctly.

Medium Engineer only £90-180

Parts commonly replaced

  • NTC Temperature Sensor
  • Sensor Wiring Harness
  • FlowSmart Control PCB
Full explanation for 12
16

Your boiler has stopped working because the fan that safely clears away used gas fumes is not running correctly.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Venturi tube
Full explanation for 16
20

Your boiler has failed to detect a flame when it should be running, causing the system to shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • PCB (Printed Circuit Board)
Full explanation for 20
23

Your boiler has stopped working because one of the internal sensors that tracks the temperature of the water returning to the unit has developed a fault.

High Engineer only £100-180

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness
  • Sensor terminal connection

Related codes

Full explanation for 23
24

Your boiler's control panel has detected that one of the buttons is stuck or faulty, preventing you from changing settings.

Medium DIY-safe £120-280

What to check first

  1. Identify if any buttons on the front panel feel stuck or recessed.
  2. Gently press and release every button on the control panel to see if they click back into place.
  3. Gently wipe the control panel with a dry cloth to remove any debris around the buttons.
  4. Restart the boiler using the main power switch or reset button.

Parts commonly replaced

  • Control Panel Interface
  • Main PCB
Full explanation for 24
27

Your boiler is struggling to move water around the system correctly, which is causing it to shut down to prevent overheating.

High DIY-safe £100-£350

What to check first

  1. Check that your boiler pressure is between 1.0 and 1.5 bar on the gauge; top up if it is low.
  2. Ensure all radiator valves are fully open and not turned off.
  3. Listen for air in the system and bleed your radiators if they have cold spots.

Parts commonly replaced

  • Circulation pump
  • Primary flow sensor
  • Main heat exchanger (flushing required)

Related codes

Full explanation for 27
29

Your boiler has detected a problem with the sensor that monitors exhaust gases, and it has shut down as a safety precaution.

High Engineer only £120-£220

Parts commonly replaced

  • Flue NTC sensor
  • Wiring harness
  • Main PCB
Full explanation for 29
31

Your boiler has lost its connection to the main thermostat or control panel, meaning it cannot receive instructions to turn the heating on.

High DIY-safe £90-180

What to check first

  1. Check if the batteries in your Alpha Climatic room thermostat need replacing
  2. Ensure the thermostat is within signal range of the boiler
  3. Restart the boiler using the reset button to see if the connection restores

Parts commonly replaced

  • Alpha Climatic Remote Control
  • Receiver PCB
  • Wiring harness
Full explanation for 31
37

Your boiler is not receiving enough electrical power to operate correctly, which is often caused by a temporary dip in the mains electricity supply.

High DIY-safe £0-120

What to check first

  1. Check if there is a power cut or brownout in your local area
  2. Turn the boiler selector switch to 'Reset' for 5 seconds and then back to 'On'
  3. Ensure your household fuse box hasn't tripped and that the boiler's fused spur is securely switched on

Parts commonly replaced

  • None (usually a temporary external power issue)
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 37
38

Your boiler has stopped working because it cannot confirm that the gas burner is staying alight, even if it has fired up initially.

High Engineer only £90-240

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • PCB
Full explanation for 38
Showing 41–50 of 57
43

Your boiler is failing to detect the flame it has lit, causing the system to shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Printed Circuit Board (PCB)
Full explanation for 43
45

Your boiler is overheating because the water is not flowing through the system quickly enough to take away the heat.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure there is enough water pressure shown on the boiler gauge (usually 1.0 to 1.5 bar)
  3. Bleed all radiators to remove trapped air which may be blocking flow

Parts commonly replaced

  • Circulation pump
  • Main heat exchanger
  • Thermistor (temperature sensor)

Related codes

Full explanation for 45
46

Your boiler has stopped running because an external temperature safety switch or a specific smart control has sent a signal to shut it down.

High Engineer only £90-180

Parts commonly replaced

  • External limit thermostat
  • Wiring harness
  • Control board connector

Related codes

Full explanation for 46
47

Your boiler is still working but cannot reach its full power, meaning your radiators might not get as hot as they should or your hot water might take longer to warm up.

Medium Engineer only £120-280

Parts commonly replaced

  • Gas valve
  • Main PCB
  • Burner

Related codes

Full explanation for 47
49

Your boiler has detected that the water returning from your radiators is too hot, causing the system to shut down for safety.

High DIY-safe £120-£350

What to check first

  1. Ensure all radiator valves (TRVs) are fully open
  2. Check that the central heating pump is running and not stuck
  3. Bleed all radiators to remove trapped air

Parts commonly replaced

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

Related codes

Full explanation for 49
51

Your boiler has lost its wireless connection to the remote thermostat or control unit.

Medium DIY-safe £0-180

What to check first

  1. Check if the batteries in your room thermostat need replacing
  2. Ensure the thermostat is within range of the boiler and not blocked by large metal objects
  3. Try resetting the boiler and re-pairing the wireless controller according to the manual

Parts commonly replaced

  • Batteries
  • Climatic RF Receiver
  • Wireless Room Thermostat
Full explanation for 51
60

Your boiler's internal pump is unable to circulate water around the system, meaning your heating and hot water will not work.

High DIY-safe £220-450

What to check first

  1. Turn the boiler power off and back on to attempt a reset
  2. Gently tap the side of the pump casing (if accessible) to dislodge any internal blockages
  3. Ensure your system pressure is topped up to between 1 and 1.5 bar

Parts commonly replaced

  • Circulating Pump
  • Pump Capacitor
  • PCB

Related codes

Full explanation for 60
61

Your boiler has detected trapped air or a blockage that is preventing water from flowing properly through the heating system.

High DIY-safe £80-250

What to check first

  1. Check that your boiler pressure is between 1.0 and 1.5 bar
  2. Bleed all radiators in the house to release trapped air
  3. Ensure all radiator valves and isolation valves under the boiler are fully open

Parts commonly replaced

  • Automatic Air Vent
  • Circulation Pump
  • System Filter

Related codes

Full explanation for 61
70

Your boiler has detected an issue with its internal temperature sensors or wiring, which prevents it from heating water safely.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Loom
  • Flow/Return Thermistor

Related codes

Full explanation for 70
71

Your boiler has stopped because the water inside isn't heating up as expected, usually due to a lack of circulation or a blockage.

High DIY-safe £120-350

What to check first

  1. Check that the boiler pressure is between 1.0 and 1.5 bar
  2. Ensure all radiator valves and isolation valves are fully open
  3. Gently bleed air from your radiators to remove any trapped pockets

Parts commonly replaced

  • Circulation pump
  • Primary heat exchanger (de-sludge)
  • NTC Temperature sensors

Related codes

Full explanation for 71
Showing 51–57 of 57
75 / 76

Your boiler is struggling to accurately measure its own temperature, meaning it cannot safely or effectively heat your water.

High Engineer only £100-180

Parts commonly replaced

  • Flow NTC Temperature Sensor
  • Return NTC Temperature Sensor
Full explanation for 75 / 76
FE94

Your boiler’s control panel has developed an internal electrical fault and is unable to communicate with the rest of the system.

High Engineer only £180-320

Parts commonly replaced

  • Display PCB
  • User Interface Ribbon Cable

Related codes

Full explanation for FE94
FE95

Your boiler has detected that an external temperature sensor is sending an incorrect or impossible reading, which is preventing the system from operating correctly.

High Engineer only £90-180

Parts commonly replaced

  • External Flow Sensor
  • Sensor Wiring Harness
  • Main PCB

Related codes

Full explanation for FE95
FE96

Your boiler is struggling to accurately measure the temperature outside, which can cause your heating to behave unpredictably or stop working altogether.

Medium Engineer only £90-180

Parts commonly replaced

  • External weather compensation sensor
  • Sensor wiring
Full explanation for FE96
FE97

Your boiler's internal computer boards are having trouble communicating with each other, meaning the system doesn't know which specific model it should be operating as.

High Engineer only £120-450

Parts commonly replaced

  • Main PCB
  • Display PCB
  • PCB Coding Chip
Full explanation for FE97
No lights on

Your boiler has lost all power and the control panel is completely blank, meaning it cannot operate at all.

High DIY-safe £100-£350

What to check first

  1. Check if other appliances in your home are working to rule out a general power cut
  2. Check your main consumer unit (fuse box) to see if the heating circuit breaker has tripped
  3. Ensure the fused spur switch next to the boiler is turned on and check the 3-amp fuse inside it

Parts commonly replaced

  • Internal PCB fuse
  • Main Printed Circuit Board (PCB)
  • External Fused Spur
Full explanation for No lights on
SR

Your boiler is simply letting you know that it is due for its annual safety check and routine maintenance service.

Low Engineer only £80-£120

Parts commonly replaced

  • None (Standard service only)
  • Burner seal (if required during service)
Full explanation for SR