Well, I don't have p0171 any longer. Driven for 150 miles. Car drives smooth. Mpg is 45mpg 2-3mpg better after I replaced CAT. Valve cover gasket was replaced 6 months ago. As expected of high mileage Prius I top off oil (1 quart) between oil changes. There are no visible oil leaks. The bolt of the part below the maf was loose (maybe when I went there to replace MAF..bcoz i got p0171 right after). That's what the mechanic said and I think that was why I was getting p0171