Equally as important as the correct scanner for your needs, is knowledge of OBDII. There are plenty of on-line articles on OBDII.
I had an extreme advantage in spite of not a great amount of car repair experience compared to our top guns on Car Talk. I worked for many years on high technology for military aircraft. The black boxes often had built in self-monitoring circuitry to report promptly any internal failures in the black box so the pilot would know not to trust his readings.
When I started reading about OBDII I realized the concepts were the same. A complex program which monitors a wide range of components and outputs, under stated conditions and within stated specs, then calls it a pass or a fail.
The engine computer does a lot, and does it very fast.
A given test will first make sure the system is ready for that test. If the sensors are not working, the computer may go into default mode, often called limp mode, but will not run other tests. So, you could have one test that fails thus masking other system problems because the other tests will depend upon the failed test as a pre-condition to run the later test.
A test may only be run at a certain minimum engine temperature, or outside air temperature.
I had a failure back in the Midwest last month. The heater on bank one sensor one opened up. That apparently is an instant CEL. Hot filaments usually fail at the instant they are turned on, like light bulbs. So, when we stopped at a truck stop to get souvenir post cards, the light came on the instant I started the car. I noted it as soon as we re-entered the Interstate, and stopped at the first rest stop to read the code.
It was only the heater, so after warm-up which doesn’t take that long, the injection was normal. I was able to continue the cross country trip to McAllen. The big danger of driving with a known CEL is if you get another one, you won’t know it.
Other tests may not run very often at all.
Evap pressure is a tricky one. A lot of conditions have to be met before it will even run the test at all. And, if it fails, the codes tell you almost nothing about what caused it. In the case of my Sienna, it was very intermittent, and another man on Sienna Chat swapped his canisters between his second Sienna and fixed it, so I knew what the problem was.
My point is not to supply an OBDII primer, but to explain why you need to learn about OBDII. For first timers, the concepts can be scary, and yet once you have a basic grasp of the concepts the scanner is much more useful.