I am making this thread because others maybe having a similar problem of finding a solution difficult. I own a 2008 Volvo S80 T6 and I was getting a P0717 error code every 150 to 300 miles after erasing the error code. So the code took enough drive cycles to reappear. I searched online and looked at various u-tube videos dealing with this problem. I started with the less expensive route of cleaning the MAF sensor with appropriate cleaning fluid. Vacuum cleaned the air filter with a shop vacuum and a visual inspection of the air lines around the engine. Borrowed a basic scan tool that gave long term fuel trim and short term fuel trim values as well as fuel rail pressures. I decided I would make a smoke machine to detect vacuum leaks that I would not be able to see instead of spraying various fluids around the motor and listening for idle speed changes. This is the website I used because it includes where he bought his material. So I made the smoke machine and began pumping the smoke into the air intake after the MAF sensor. Eventually smoke started to come out from under the oil fill cap. I opened the oil fill cap and noticed an o-ring that was responsible for sealing the cap had hardened due to hundreds and hundreds of heat cycles. I ordered an OEM o-ring from Ebay. I tried to find a replacement at the local hardware store. ($.69) and noticed the error code went away. When the OEM o-ring came ($6.00) replaced the hardware store o-ring. NO more error code! Can a leaking oil filler cap cause this error code? YES it can. Never saw this as a listed cause of P0171. Hope this helps someone else searching for a reason for this error code. I want to thank this community of people who are trying to maintain their cars in a cost effective manner. I have learned from others how to overhaul the OAC and rear differential and repair the blower motor in this car. The wealth of knowledge out there is impressive and growing. P.S. motor idles smother and was getting 16. something mpg and now getting 20 something mpg. Leave a comment if this helped you find your problem.