Thank you for your question.
My name is Steve and I am a fully qualified vehicle technician with over thirty years in the trade. Hopefully I can help you.
The problem you have sounds as though it is related to a poor earth somewhere in the system. This may be from the battery or somewhere else on the vehicle.
The scenario you have where the engine will run with the diagnostic connector is one that is quite common and one I have come across before. This will not always leave a fault code in the system.
The reason that the engine will run when the diagnostic scanner is connected is because this then gives a route to earth for the required components.
As the vehicle has been laid up it may be corrosion on any of the ECU terminals or engine earth terminals.
The computer the mechanic used should indicate any other problems such as low fuel pressure or crank sensor and so we should be able to eliminate these.
If the problem is not an earth one there should be a fault code of some sort in the ECU memory, and your mechanic should be able to access this using his computer.
I hope this answers your question, but do please ask me for any further details.
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