I have yet to see a “code reader” or smartphone app that I find useful in the daily repair of cars. Sure grabbing a fault code and having some (not all) live data can start you, but I need much more than any of the devices I see talked about here to be effective and efficient at my job. I need to be able to drive the car and see all the data I find to be pertinent, perhaps graph the inputs from several sensors while driving, see multiple lines of data, and so on. If nothing else, a smartphone simply isn’t big enough to have that on a screen that’s easily read. I need to able to disable fuel injectors, run emissions systems tests, operate cooling fans, shift a transmission into 4th gear at 10mph, etc. To the best of my knowledge you need an actual “scan tool” to perform all these functions.
For example: Last week I had a car in with a complaint of no A/C. The fan blows warm air. I didn’t need to open the hood, or install pressure gauges. 90% of the diagnostic process was done from the driver seat using nothing but a scan tool. I was able to determine that the car had a leak and was low on refrigerant by reading scan tool data, and was also able to identify a faulty under-dash sensor at the same time.
Same day we had a car in for rear brake replacement. A scan tool was needed to retract the parking brake mechanism. Also at the same time we used it to reset the oil life monitor.
Do any of the code readers in the posts above do that much? Not sure, but I doubt it.