I tend to agree with Steve Norman that eventually specialisation in one or maybe two related makes may be the only way to go. On the subject of diagnosing faults, as some have said, diagnostics are not simply confined to the electronic systems. A lot of vehicle equipment is electro-mechanical or electro-hydraulic. The ability to pinpoint and recognise mechanical faults or fluid leaks in engines, transmissions, brake systems and body components is just as important. A high degree of mechanical knowledge and previous experience is often required to make sense of what various tools such scanners, 'scopes and vacuum gauges are telling you. It's all very well diagnosing say, a wheel speed sensor failure.If it's corroded solidly to the housing, some degree of mechanical skill will be needed to replace it in a timely manner.Perhaps the sensor is fine and it's a faulty hub bearing, then you may need a hydraulic press. The customer won't want it diagnosed at one garage and repaired at another, he wants a one-stop shop.