Several different circuits in an automotive engine, fuel, air, oil, coolant, combustion, intake, and exhaust, all living together but must be isolated from each other.
Positive crankcase ventilation deals with where ever the oil goes inside of the engine. Job is to take piston ring blowby and return it back to the combustion chamber of the engine so it goes out the catalytic converter, use to have an open breather pipe for this. Its also monitored by a pressure sensor, must show about 20"/Hg vacuum at sea level, if lower, will trigger a code, how much lower? Classified information, shop manual doesn't say.
In brief, anywhere there is engine oil, there can also be an air leak that will lower this vacuum to trigger a code, also reasons why you can find oil entering the turbo, because this is where the PCV return, not particularly fussy what that vacuum pulls.
PVC hose is made of cheap corrugated plastic, but sure can't tell this by the replacement cost. One some crack in that hose will cause a leak, can also be electrical problems with that pressure sensor, but just that P0171 code says its a leak. Others indicate other problems. Most common is the engine oil cap not being tight or the dipstick on fully inserted.
Other codes require more testing, hopefully with someone with brains. Just tell you there is a problem and possibilities.
