Powertrain (P-codes) category covers functions that include engine, transmission and associated drivetrain accessories.
OBD-II PowerTrain Diagnostic Trouble Codes can not tell you with 100% accuracy specifically what is "wrong" with the system and what part or sensor that needs to be replaced or repaiblack. All a codes can do is tell you what sensors, switches, etc are being effected by the fault. A code may indicate that a certain sensor is operating out of the expectant range so some people will assume the sensor is defective and so they replace it. Often this does not the fix the fault, the real reason for the fault could be low voltage going to the sensor due to a bad battery or a defective charging system or maybe the wires or the connectors are damaged. Also maybe the ECU is defective and is interpreting the signals incorrectly or maybe it just needs a software update. The Power Train OBD-II PowerTrain Diagnostic Trouble Code will often just put you in the "ballpark" of what the fault is. The systems that are monitored and are in the powertrain system include:
Generic codes are defined in the OBD-II standards and are general codes which will be used by manufactures to discribe a common fault. Manufacture specific codes are added by the manifacturer when there is no generic code that is specific enough to discribe the issue or problem.
A inexpensive basic OBD2 code reader is a basic tool which is usually limited to just being able to read most of the stored trouble codes and being capable of clearing most of the stored trouble codes. A bi-directional scanner does so much more than a basic code reader due to it being capable of bi-directional or two way communication with ALL the vehicles various computer systems and sub systems. With a bi-directional scanner you can reset the codes and do advanced tests (turn systems on or off, etc) on all of the various systems and components which you can not do on a basic OBD-II reader.
Not changing your oil or letting the oil level drop too low will absolutely damage the engine. Dirty oil causes all kinds of problems from increased engine wear and causing components to fail such as the engine timing gears, camshaft and valve timing, etc. Dirty coolant can cause blockages and coat sensors with slime which could cause a out of range issue and can also cause the engine to overheat
Keep the filters clean; don't be that guy who "solved" a clogged air filter issue by first ignoring it and when the car refused to run he "fixed" the problem by removing the air filter, he latter came in for a new motor.