A lack of power from the battery to the starter motor can be caused by a range of things starting with the physical stuff like the cabling, terminals or ignition switch, and winding up with a fault in the immobiliser or the on-board computer, for instance. Sluggish performance, meantime, can be any number of things, and just to list them all would take hours.
So, before you go out and spend more money replacing components that may or may not be at fault, have the car scanned at a workshop and see what fault codes it throws up. From there, you’ll have a much better idea of what’s going wrong.
