Hello Mike,
Going back to the good old days of mk1/2 escorts and hillmans and BL offerings etc. The mobile diagnostics of the day or even garage operators could keep a few HT leads and a long king lead, half a dozen dizzy caps few coils, plugs etc and hey presto you've covered alot of vehicles. People probably thought cars were more complex mechanical machines. Nowadays most of the people i have delt with have moved into much more complex vehicles, i.e one person I know,started with an Austin Maxi and over the years up to a Mercedes S320 BUT he thinks its much easier now because,- His words "you just plug it in and it tells you". Bare in mind Maxi dianostic equipment, bit of wire and a bulb, Fits all makes (if you was flash a voltmeter and strobe light) and you could make a bob or two, car working. NOW to the Merc same costomer Merc diagnostic equipment, well how deep are your pockets what do you want it to do? you invest and hope over time you will see enough Mercs to get you money back and then try to make a bob or two, but the customer has probably in real terms paid less for the repair now than he would have done years ago but they think you are having a holiday on them! Because they have a mate at the Golf club who has a mate who has the latest diagnostic gear that can do EVERYTHING for £9.99 of the e-bay!!!AND this is only one example ("O" look outside the workshop BMW X5 and a Citroen Saxo wheres me multi purpose king lead gone!!or where is the man from the golf club with his E-Bay £9.99 silver bullet machine) Then you find yourself spending more time explaining the truth over the myth but by then the customer glazes over and just thinks you've lost the plot. (Rather like this book what I have rit).

Answer to your question>>>>>No I don't think you could make a living only doing diagnostics, as you could have done not so many years ago, its just far too complex and the general public don't understand what their cars have become

"The Hampster wheel gets bigger and we run faster just to stand still"

"Its Not Easy,"

Ian.