Common problems guide

The most common Sime Murelle Pro HE MkII Combi problems

The faults most likely to send a Sime Murelle Pro HE MkII Combi into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

20 documented codes
6 most common
1 DIY-safe checks
5 engineer needed

The 6 most common faults

01
AL 98 High DIY-safe

Your boiler has encountered a digital 'glitch' or software error while trying to start up, preventing it from running safely.

What to try first

Locate the selector switch on the front of the boiler control panel.Turn the dial to the 'Reset' position for 5 seconds.Turn the dial back to the 'Summer' or 'Winter' setting and wait for the boiler to restart.

Est. cost
£180-350
Parts
Main Printed Circuit Board (PCB)
Full guide for AL 98
02
AL 17 High Engineer

Your boiler has detected that its two internal temperature sensors are giving very different readings, suggesting a potential circulation issue or a faulty component.

Est. cost
£120-240
Parts
NTC Heating Sensor, Dual Sensor Assembly, Main Heat Exchanger
Full guide for AL 17
03
AL 30 High Engineer

Your boiler has detected a technical issue with the sensor that monitors water temperature returning to the unit, causing it to stop working for safety.

Est. cost
£90-160
Parts
Return temperature NTC sensor, Sensor wiring harness
Full guide for AL 30
04
AL 44 High Engineer

Your boiler has stopped working because it cannot detect a flame, which is likely due to an internal electronic fault or a problem with the gas supply valve.

Est. cost
£180 - £450
Parts
Gas Valve, PCB (Main Control Board), Ignition Electrode
Full guide for AL 44
05
AL 56 High Engineer

Your boiler has shut down because it detected a dangerous temperature difference between the water leaving the boiler and the water returning to it, suggesting it is overheating.

Est. cost
£120-450
Parts
Circulation pump, Main heat exchanger (de-scaling), Flow/Return thermistors
Full guide for AL 56
06
AL 57 High Engineer

Your boiler has detected a temperature mismatch and has locked itself to prevent overheating, requiring a system recalibration to restart.

Est. cost
£90-180
Parts
Flow Temperature Sensor, Main PCB, Internal Pipework Insulation
Full guide for AL 57
Parts most often replaced

on the Sime Murelle Pro HE MkII Combi

Main Printed Circuit Board (PCB)NTC Heating SensorDual Sensor AssemblyMain Heat ExchangerReturn temperature NTC sensorSensor wiring harness

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Sime shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
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See the full 20-code list for the Sime Murelle Pro HE MkII Combi

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →