P0171 and P1101 error codes. What are they and how to troubleshoot them? Chevrolet Cruze

We have a P0171 error code that is often accompanied by P1101 or other codes. Code means that computer is unable to maintain the correct mixture of air and fuel in the cylinder causing rough idling or in some cases loose power. We have used Foxwell OBD reader. This was a very quick troubleshooting as we could hear the hissing from PCV valve. PCV valve is welded on cover so there is no easy fix but to replace the whole valve cover.