Many years ago I made the mistake of going out to intermittent faults and not invoicing if I couldnt find the fault (which wasnt present at the time)
Some times you will 'cure' the fault just by unplugging a component (classic is air bag seat looms) or moving a loom away from a short.
I would say ring me back if it occurs again and 90% didnt
so I've learnt to charge at that point.
there is however a feeling that you are now committed and the job(problem)is yours forever, but on balance you will gain financially by always charging for the first look-if you dont invoice immediatley always ring in a week ( I've lost a few quid by being too busy and forgetting)

If I were wrrrrrrrong in a diagnosis I would stand the cost of the incorrectly diagnosed part and put it on the shelf and down to experince. It is important to let the customer understand that you are fitting a cam sensor because the scanner suggests it is faulty but is no guarantee of a cure-just the first step and that is what the main dealer would do-if he would prefer to go there
Regards
Barry