Vokera Compact 24 - starting and stopping continuously



I've read various posts on here relating to the vokera Compact 24 and the issues with it. Mine lights up, runs for a second or two then turns off, it repeats this process for 5 mins then it may light and stay lit or it may not.
I have changed the 2 thermistors (red and blue) and the high temp sensor on the side of the heat exchanger as well as the air pressure switch, oh and the DHW actuator.The flue looks fine and the vacuum pipes are perfectly clean. The system has been in place for 5 or 6 years with no issues. I periodically clean out the heat exchanger and the system always has fernox inhibitor in it.

Is there any thing else I can try before I bite the bullet and call someone in?

Many Thanks in advance for your assistance.

Forgot to say it does this for the hot water and the heating.
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Mclearg areyou gas safe registered?, if you're not then don't open up the combustion chamber. If you've changed the thermistors, then i would get on to a gas safe registered engineer.
No, I'm not but i haven't disturbed the gas at all so i didn't think I was causing an issue. The thermistors I changed are on the water lines and the surface of the heat exchanger - no gas lines disturbing.

I'm a mechanical engineer as it happens.
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There is no issue with the combustion , i'm pretty sure it's a sensor at fault as it will light on most occasions and does not lock out. The ignition is fine , it's the process afterwards that is crapping out. i may deconstruct the flue next although there is no obvious damage/wear.
I will call an engineer but only after i feel i have checked everything i can. Thanks for your reply but do you, or anyone else have any actual advice?
I dont think anyone will give you advice if your on about dismanteling flues etc, you say sensor fault, but the only sensors on that boiler you've already changed. You should just bite the bullet and call out someone qualified.

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