Yes it is a daft question!
If the engine cranks ok, without petering out within 15 seconds then i can already confirm, it has nothing to do with the battery. Applying jump leads will make no difference.
I agree with the mechanic about maybe a possible earth fault. However, its very easy to get misled. Start with the basics as 9 out of 10 times its nearly always something simple.
Battery voltage above 12.5v before crank. If below 12.2v then you need to charge and test! Any battery showing voltage below 12.2v is technically Discharged.
If engine cranks ok, check for rpm signal. Should be 200-300rpm on 'live readings' when cranking.
If you have an rpm signal then one must assume (at this point that you have a spark, switching at the injectors and 4-6amps draw on the fuel pump circuit.
you must also have 12v at the MAF and also 5v at the MAF.
(If any of these are not present, then the car will never start!!)
12v means that the main relay is powered up and live and 5v means that the CEM/ECM are live and powered up.
A bad earth will give you sporadic faults and intermittent symptoms - but they will be easily identifiable on AUTODATA. It will list the Earth points on your car and thus, your mechanic bloke should be able to check resistance at each one. Any voltdrop above 0.5v will indicate a bad earth and a potential problem.
Undo said earth point, derust, wirebrush and electro lube it, then retighten.
then check if the car starts again.
If your mechanic is worth his salt, then he'll know all this anyway.
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