Car won’t start sometimes, make click sound when turn key.
Mechanic says nothing wrong with starter, trans, battery, switch.
What is wrong?
The mechanic/
The first thing I would check is that the battery posts and terminals are clean and free of corrosion.
Not just on the outside…you have to remove the terminal and clean both mating surfaces.
The next is to check the cables themselves. It being 15 years old those cables may be corroded beyond use under the insulation. Use a knife and peal back an inch of insulation and check.
I’ve taken off cables that that just crumbled when you bent them in half.
Yosemite
An intermittent no-start condition can be caused by a faulty starter relay.
https://www.rockauto.com/en/moreinfo.php?pk=444545&cc=1410534&jsn=422
When the mechanic tested the starting circuit at the time, the relay could have been working.
Replace the relay.
It isn’t expensive. And it’s taken out of the equation as to why the engine doesn’t start sometimes.
Tester
You have to also check the other end of the cables, the one that goes to the starter and the one that goes to ground, ie, is bolted to the chassis of the car.