Most workshop have one or a few scan tools that will diagnose a vehicle when the MIL light is on. and DIYers/car owners may also have a OBD scan tool. When the MIL light is on, you can use a OBD scan tool like Launch X431, Autel, OTC, Snap-on,alfatest, technomotor, motorscan, innova etc(there are more than enough diagnostic equipment manufacturers). and the result is mostly acceptable, though in most cases, different scan tools might get different result.
But had you ever encounter mulfunction of the engine without MIL light ON? you feel that you car works not normal, hard to startup, more fuel consumption, not as powerful as when it was bought, and this give you a big headache, you bring the car to garage or workshop, they can not find out what's wrong with it, but you know its different.
We are going to talk about this issue,
There's no MIL light on, because all sensors are working good. the problem is with other parts, like dirt, or sealing. but you would ask, smoke machine manufacturer like J&L Techno said leak in pipe system will cause sensors to work abnormal. yes, we did said so. and we will say it again. then why there's no MIL light on?
The turth is, sensor signal won't be liner, see the graph below,
The above gragh is a knock sensor waveform, and its working in a good condition. this is because each sensor calibration have a tolerence range.
while the signal is changing within the tolerance range, the ECU won't determine it as abnormal, or else the MIL light in your car would always ON.
BUT, if the wav form exceed the range for several times, the ECU would determine it as abnormal, and put MIL light ON.
So, its pretty clear, there is something wrong, but not wrong enough to trigger the trouble code(within tolerence range), in this case, can clean the dirt and check leakage in intake system.