P0171 help!!!

For P0171 code, the two parameters that you should look at are
1. Short Term Fuel Trim = 5.47%
2. Long Term Fuel Trim = 21.88%

The total fuel trim is determined by adding the short term fuel trim with the long term fuel trim, 5.47 + 21.88 = 27.35%. If the engine is operating perfectly the total fuel trim will be 0%. An engine that is running well will have a total fuel trim of plus or minus 8% or less. A positive fuel trim means the engine is running lean. A negative fuel trim means the engine is running rich. So, your engine is definitely running lean.

As the car gets older, the vacuum hoses seem to split at the ends where they are connected. I found this to be true on my niece's 2010 Prius which had the same problem about 10 months ago. I also checked my 2011 Prius and the same two hoses had started to split on the ends, but were not leaking yet. I replace the hoses on both cars with OEM parts. Problem solved.

Take the air box off the engine and check the vacuum hoses near the throttle body and at the VSV (purge) valve. If you find cracks at the ends of the hoses, you can temporarily fix it by wrapping electrical tape around the hose end. You should then buy replacement hoses from your Toyota dealer.

Take a look at this thread, P0171 - Recently replaced pistons & rings