BoilerFaultCodesThe 6 most common faults
01
ALL 02 High DIY-safe
Your boiler has stopped working because the water pressure inside your heating system has dropped too low to operate safely.
What to try first
Locate the external filling loop, which is usually a small silver braided hose connecting two pipes under the boiler.Open the valves at both ends of the filling loop to allow cold mains water into the system.Watch the pressure gauge on the boiler front and close both valves firmly once the needle reaches the green zone, typically between 1.0 and 1.5 bar.
02
ALL 04 High Engineer
Your boiler is failing to detect the temperature of the water coming out of your hot taps, which means it cannot heat your hot water correctly.
Full guide for ALL 0403
ALL 05 High Engineer
Your boiler has stopped working because it is unable to accurately measure the temperature of the water being used for your heating system.
Full guide for ALL 0504
ALL 06 High Engineer
Your boiler has detected a failure to ignite or a problem with the flame detection process and has safely shut itself down to prevent gas buildup.
Full guide for ALL 0605
ALL 07 High Engineer
Your boiler has detected that it has become dangerously overheated and has shut itself down to prevent damage.
Full guide for ALL 0706
ALL 08 High Engineer
Your boiler is failing to light because it cannot detect a flame, so it has shut down as a safety precaution to prevent a gas build-up.
Full guide for ALL 08on the Sime Murelle Elite HE Combi
Filling loopPressure gaugePressure relief valveDHW NTC sensorWiring harnessCentral heating NTC sensor
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