faster4_tec said:
I doubt you'll find even a small garage that doesn't have one nowadays.
true, they dont cost much and any idiot can use them, so most places have them, and those who dont will either know somebody who does or will tell customers to visit the dealer.
if you want to provide the service to local garages you need to be available during business hours, willing to travel to any garage which requests it, willing to do it cheaply enough to allow the garage to make a profit (you may get £10-20 per diag if you're lucky) and you need to be able to provide the service for all modern makes and models, meaning regular expense on software updates. also you need to be able to offer a good service, a fault code isnt worth paying for, you need to be able to give a detailed diagnosis of all modules. doing that with a 3rd party tool is practically impossible, which is why so many garages refer you to the dealer for a diagnostic, they know 3rd party tools rarely work as well. if you can afford all the equipment you require to do a detailed diagnostic on all modern makes and models, how many years will it take to get that money back and actually make a profit.
to make money with a diagnostic machine you need to forget about garages, try to find people with simple car problems who cant afford the prices dealers charge for diagnostics. most small garages charge about £30 for a diagnostic, you could charge the same but provide a mobile service so when people are stuck at home with car problems they can call you. if you advertise correctly this can be a worth while thing to do, but it wont be a full time job, or enough for you to live off, so you'll need another source of income, one that will still allow you to be available for your mobile diagnostic service. thats difficult.
self employed mechanics make their money doing the easy stuff like servicing, MOT prep, and exploiting daft boyracers who want one off mods and ICE installations. i made very good money doing that for a few years. diagnostic services would have been a good addition to my services (its also quick and easy), and it is something i considered and investigated (even made my own tool back in 2001), but i found the affordable 3rd party diagnostic tools were not reliable or universal enough to make them worth while and pay for themselves, so I never bothered. only offering a diagnostic service would seriously limit the amount of business you could do. its only the sort of thing a self employed or mobile mechanic should add to the rest of their services, its not something you can realistically make a real profit from alone.