Sime Mia 30 Fault codes & diagnostics

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

Combi Natural GasLPG 30 kW 2022-present
GC Number 47-283-90
Download the Sime Mia 30 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.
2 Emergency 43 High 3 Medium 2 Low
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-283-90 MIA- 30 2022–present
Just want the most common Sime Mia 30 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 50 documented codes

Tap any card for details

02

Your boiler has stopped working because the water level 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 one or two taps on the hose until you hear water entering the system.
  3. Watch the pressure gauge and close the taps once the needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 02
06

Your boiler has failed to light the gas or cannot tell that the flame is burning, so it has shut down for safety.

High DIY-safe £100-280

What to check first

  1. Check if other gas appliances (like a hob) are working to ensure your gas supply is active.
  2. If you have a prepay meter, check that you have sufficient credit.
  3. Press and hold the Reset button for a few seconds to try restarting the boiler.

Parts commonly replaced

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

Related codes

Full explanation for 06
07

Your boiler has shut down because it has detected an unsafe build-up of heat, often caused by a blockage or poor water circulation.

High DIY-safe £100-280

What to check first

  1. Check if any radiator valves are closed and open them fully.
  2. Ensure the boiler pressure is between 1 and 1.5 bar on the gauge.
  3. Attempt to reset the boiler once it has cooled down.

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Primary Heat Exchanger

Related codes

Full explanation for 07
AL

Your boiler has detected a general problem and is waiting for a specific sub-code to appear to tell you exactly what is wrong.

High Engineer only £100-350

What to check first

  1. Check if a more specific two-digit number appears after the AL code
  2. Try resetting the boiler by turning the selector dial to the reset position for five seconds
  3. Ensure your gas supply is active and credit is on the meter

Parts commonly replaced

  • Ignition Electrode
  • Printed Circuit Board (PCB)
  • Fan

Related codes

Full explanation for AL
AL 01

Your boiler has detected that fumes are not venting correctly and has safely shut down to prevent the build-up of dangerous gases in your home.

Emergency Engineer only £150-300

Parts commonly replaced

  • Flue thermostat
  • Flue assembly seals
  • PCB

Related codes

Full explanation for AL 01
AL 02

Your boiler has stopped working because the water pressure inside the heating system has dropped below the minimum level required to operate safely.

High DIY-safe £80-150

What to check first

  1. Locate the external filling loop, which is a silver braided hose connecting two pipes under the boiler.
  2. Open the small valve or tap on the loop to allow water into the system, watching the pressure gauge on the front of the boiler.
  3. Close the valve firmly once the needle on the gauge reaches between 1 and 1.5 bar, then check the boiler for the fault code to clear.

Parts commonly replaced

  • Pressure relief valve
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for AL 02
AL 04

Your boiler is having trouble measuring the temperature of your hot water, which usually means your taps will only run cold.

High Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) thermistor
  • NTC sensor wiring harness

Related codes

Full explanation for AL 04
AL 05

Your boiler is notifying you that a fault occurred previously, though it may not be active right now.

Low DIY-safe £0-120

What to check first

  1. Press and hold the 'Reset' button for a few seconds to clear the history alert
  2. Monitor the boiler display to see if a more specific error code appears
  3. Ensure your system pressure is between 1 and 1.5 bar

Parts commonly replaced

  • None (Software reset)
  • NTC Sensor

Related codes

Full explanation for AL 05
AL 06

Your boiler has failed to ignite or stay lit, meaning it cannot produce heat or hot water.

High DIY-safe £100-280

What to check first

  1. Check if other gas appliances (like a hob) are working to ensure gas supply is active
  2. Ensure your prepaid gas meter has credit if applicable
  3. Press and hold the 'R' or reset button for a few seconds to restart the ignition sequence
  4. Check that the condensate pipe outside hasn't frozen in cold weather

Parts commonly replaced

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

Related codes

Full explanation for AL 06
AL 07

Your boiler has detected that it has become dangerously hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Safety thermostat
  • Pump
  • Diverter valve
  • Heat exchanger

Related codes

Full explanation for AL 07
Showing 11–20 of 50
AL 08

Your boiler is notifying you that a previous fault occurred in the past, even if the system is currently working.

Low DIY-safe £0-80

What to check first

  1. Press and hold the 'Reset' button for 1-3 seconds to clear the notification
  2. Observe the boiler to see if a different, active fault code appears
  3. Check that your system pressure is between 1.0 and 1.5 bar

Parts commonly replaced

  • None (Software reset usually sufficient)
  • Faulty sensor (only if a secondary code persists)

Related codes

Full explanation for AL 08
AL 09

Your boiler has detected that water is not flowing through your heating system as it should, so it has shut down to prevent damage.

High DIY-safe £120-250

What to check first

  1. Check your radiators and under-boiler pipes for any obvious leaks.
  2. Check the pressure gauge on the front of the boiler; if it is below 1 bar, use the filling loop to top it up to between 1 and 1.5 bar.
  3. Ensure all radiator valves and any isolation valves near the boiler are fully open.

Parts commonly replaced

  • Circulation pump
  • Flow switch
  • System filter

Related codes

Full explanation for AL 09
AL 10

Your boiler has detected an issue with a secondary sensor used for monitoring temperature, which has caused the system to stop working as a safety precaution.

High Engineer only £90-180

Parts commonly replaced

  • Auxiliary temperature sensor
  • NTC sensor
  • Wiring harness connector

Related codes

Full explanation for AL 10
AL 11

Your boiler has detected a communication problem with the component that controls the gas supply, meaning it cannot safely light or adjust the flame.

High Engineer only £120-£350

Parts commonly replaced

  • Gas Valve Modulator
  • Gas Valve Assembly
  • Wiring Harness

Related codes

Full explanation for AL 11
AL 12

Your boiler has lost communication with the temperature sensor that monitors your hot water supply, meaning it cannot heat water to the correct temperature.

High Engineer only £90-160

Parts commonly replaced

  • DHW Temperature Sensor (NTC)
  • Sensor wiring harness

Related codes

Full explanation for AL 12
AL 13

Your boiler has detected that the exhaust gases are getting too hot and has shut down as a safety precaution.

High Engineer only £120-280

Parts commonly replaced

  • Exhaust sensor (SF probe)
  • Main Heat Exchanger
  • Fan

Related codes

Full explanation for AL 13
AL 14

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 £90-180

Parts commonly replaced

  • Exhaust sensor (SF)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for AL 14
AL 15

Your boiler has stopped working because the internal fan, which safely exhausts fumes, is not communicating correctly with the main control board.

High Engineer only £120-280

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Main PCB

Related codes

Full explanation for AL 15
AL 18

Your boiler has stopped working because the waste water pipe is blocked, likely due to a clog or an ice buildup if it is cold outside.

High DIY-safe £80-160

What to check first

  1. Check the white plastic pipe exiting the boiler for any obvious blockages or kinks.
  2. If the weather is freezing, pour warm (not boiling) water over the external section of the pipe to melt any ice.
  3. Reset the boiler by turning the selector dial to the 'R' position for a few seconds.

Parts commonly replaced

  • Condensate trap
  • Condensate level sensor
  • External discharge pipework

Related codes

Full explanation for AL 18
AL 28

Your boiler has been manually reset too many times in a short period and has now locked itself for safety.

High DIY-safe £80-120

What to check first

  1. Turn off the power supply to the boiler at the fused spur switch
  2. Wait for at least 30 seconds
  3. Turn the power back on to clear the software block
  4. Do not attempt further resets if the original fault persists

Parts commonly replaced

  • No parts usually required for this specific code
  • Ignition Electrode
  • Gas Valve

Related codes

Full explanation for AL 28
Showing 21–30 of 50
AL 30

Your boiler has stopped working because its internal thermometer, which measures the temperature of the water returning to the unit, has developed a fault.

High Engineer only £100-180

Parts commonly replaced

  • Return NTC thermistor sensor
  • Sensor wiring harness

Related codes

Full explanation for AL 30
AL 37

Your boiler has detected that the electricity supply from the mains is too low for it to operate safely and has shut down to protect its components.

High Engineer only £0-250

Parts commonly replaced

  • Main PCB
  • Voltage Regulator

Related codes

Full explanation for AL 37
AL 40

Your boiler is failing to light the flame or detect that it has started, so it has shut down as a safety precaution.

High Engineer only £120-220

Parts commonly replaced

  • Ignition electrode
  • Flame rectification probe
  • Ignition lead

Related codes

Full explanation for AL 40
AL 41

Your boiler has tried to light several times but the flame keeps going out, so the system has locked itself for safety.

High Engineer only £120-350

Parts commonly replaced

  • Ionisation electrode
  • Gas valve
  • Ignition lead
  • Condensate trap

Related codes

Full explanation for AL 41
AL 42

One of the buttons on your boiler's control panel has become stuck or is malfunctioning, which is preventing you from changing settings.

Medium DIY-safe £100-250

What to check first

  1. Check all buttons on the front panel to see if any are stuck in the 'pushed' position.
  2. Gently press and release each button several times to ensure they move freely.
  3. Turn the boiler power off and back on at the fused spur switch to reset the electronics.

Parts commonly replaced

  • Control Interface Board
  • Main PCB
  • Button membrane

Related codes

Full explanation for AL 42
AL 43

Your boiler is having trouble communicating with your smart thermostat or external heating controls, which means it may not know when to turn on or off.

Medium DIY-safe £90-180

What to check first

  1. Check that your smart thermostat or wall controller is powered on and has working batteries.
  2. Ensure any wireless receiver boxes plugged into or near the boiler have power lights on.
  3. Turn the boiler off and back on at the main fused spur switch to refresh the connection.

Parts commonly replaced

  • OpenTherm Interface Cable
  • External Thermostat/Controller
  • Boiler PCB

Related codes

Full explanation for AL 43
AL 44

Your boiler is failing to detect a flame because the gas valve is not opening properly or within the required time.

High Engineer only £120-280

Parts commonly replaced

  • Gas valve
  • Ignition electrode
  • Control board

Related codes

Full explanation for AL 44
AL 56

Your boiler has detected that the water is heating up too quickly or not circulating properly, causing it to shut down to prevent overheating.

High DIY-safe £100-280

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 which may be blocking flow

Parts commonly replaced

  • Circulating pump
  • Return temperature sensor
  • Main heat exchanger (flushing)

Related codes

Full explanation for AL 56
AL 57

Your boiler has sensed a problem with the water flow temperature, potentially due to air in the system or a blockage, and has shut down as a safety precaution.

High DIY-safe £100-250

What to check first

  1. Check that your system pressure is between 1 and 1.5 bar on the pressure gauge
  2. Bleed your radiators to remove any trapped air
  3. Attempt to reset the boiler using the 'R' or reset button
  4. Ensure all radiator valves are fully open

Parts commonly replaced

  • Flow temperature sensor (NTC thermistor)
  • Circulation pump
  • Automatic air vent

Related codes

Full explanation for AL 57
AL 62

Your boiler's electronic control system needs to re-learn how to burn gas efficiently and is currently unable to operate until this calibration is complete.

High Engineer only £80-250

Parts commonly replaced

  • Gas valve
  • Electrode
  • Main PCB

Related codes

Full explanation for AL 62
Showing 31–40 of 50
AL 72

Your boiler has detected that a temperature sensor is not in the correct position or is sending an unusual reading, preventing it from heating up safely.

High Engineer only £90-180

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Sensor clip
  • Wiring harness

Related codes

Full explanation for AL 72
AL 74

Your boiler has detected a problem with one of its internal temperature sensors, meaning it cannot accurately monitor the heat of the water leaving the unit.

High Engineer only £90-180

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for AL 74
AL 77

Your boiler has detected an electrical issue with the gas valve control system, which prevents it from safely managing the fuel supply.

High Engineer only £180-350

Parts commonly replaced

  • Gas Valve
  • Main PCB
  • Wiring Harness

Related codes

Full explanation for AL 77
AL 79

Your boiler has detected an electrical issue with the gas control valve, preventing it from operating safely to provide heating and hot water.

High Engineer only £180-350

Parts commonly replaced

  • Gas Valve
  • SGV Wiring Harness
  • Main PCB

Related codes

Full explanation for AL 79
AL 80

Your boiler has detected an internal communication error or a wiring problem with the gas valve, preventing it from heating your water safely.

High Engineer only £140-£320

Parts commonly replaced

  • Gas valve wiring harness
  • Gas valve
  • Main PCB

Related codes

Full explanation for AL 80
AL 81

Your boiler has shut down because it detected a flame at the wrong time during the starting process or the gas is not burning correctly.

High Engineer only £120-£350

Parts commonly replaced

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

Related codes

Full explanation for AL 81
AL 82

Your boiler has shut down because it has failed to ignite or maintain a flame several times in a row, often after a power cut or a gas supply disruption.

High DIY-safe £120-350

What to check first

  1. Check that your gas supply is turned on (check other gas appliances or top-up credit)
  2. Hold the R (Reset) button for 2-5 seconds until the display clears
  3. Monitor the boiler to see if it fires up; if it locks out again, call a Gas Safe engineer

Parts commonly replaced

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

Related codes

Full explanation for AL 82
AL 83

Your boiler has detected that the gas is not burning correctly, which may cause it to turn off temporarily for safety.

Medium Engineer only £100-350

Parts commonly replaced

  • Ionisation electrode
  • Gas valve
  • Burner

Related codes

Full explanation for AL 83
AL 84

Your boiler is struggling to heat water because it isn't receiving a steady supply of gas at the required pressure.

High Engineer only £100-250

Parts commonly replaced

  • Gas valve
  • Gas meter regulator
  • PCB

Related codes

Full explanation for AL 84
Showing 41–50 of 50
AL 88

Your boiler's internal computer brain has detected a hardware failure and has shut down to protect its circuits.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for AL 88
AL 89

Your boiler is struggling to maintain a steady flame, which is causing it to shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Detection electrode
  • Gas valve
  • Venturi flue assembly

Related codes

Full explanation for AL 89
AL 90

Your boiler is having trouble managing its flame correctly and has locked itself out for safety.

High Engineer only £120-£350

Parts commonly replaced

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

Related codes

Full explanation for AL 90
AL 92

Your boiler has shut down because it cannot balance the air and gas mixture properly while running at its lowest power setting.

High Engineer only £120-350

Parts commonly replaced

  • Gas valve
  • Fan
  • Venturi
  • Electrodes

Related codes

Full explanation for AL 92
AL 93

Your boiler is struggling to achieve a stable flame because the balance of gas and air is incorrect, causing it to shut down for safety.

High Engineer only £120-£350

Parts commonly replaced

  • Gas Valve
  • Venturi
  • Electrode Set
  • PCB

Related codes

Full explanation for AL 93
AL 95

Your boiler is struggling to keep its flame burning consistently, causing it to shut down for safety.

High Engineer only £120-£350

Parts commonly replaced

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

Related codes

Full explanation for AL 95
AL 96

Your boiler has shut down because it cannot safely expel exhaust fumes or draw in fresh air.

Emergency Engineer only £100-280

Parts commonly replaced

  • Flue terminal
  • Air pressure switch
  • Fan assembly
  • Venturi tube

Related codes

Full explanation for AL 96
AL 98

Your boiler's internal computer brain is having trouble starting up correctly, which is preventing the system from running.

High DIY-safe £250-450

What to check first

  1. Locate the reset button or selector switch on the front control panel
  2. Press and hold the reset button for 5 seconds
  3. Wait for the boiler to complete its restart sequence to see if the error clears

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring harness

Related codes

Full explanation for AL 98
AL 99

Your boiler's internal computer has suffered a major software or electrical failure and can no longer control the system.

High Engineer only £250-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for AL 99
ALL 91

Your boiler has stopped working because it cannot correctly control the flow of gas needed to stay alight safely.

High Engineer only £220-380

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for ALL 91
Status & diagnostic codes

2 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

AL 00 Diagnostic Your boiler is displaying the most recent fault code it recorded to help you diagnose a past issue, rather than indicating a new active problem. AL 03 Diagnostic Your boiler is notifying you that a safety fault occurred recently and the system has saved this record in its memory.
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