Been having only this code thrown intermittently for a few months now. If @ above (or just below increasing up to) 3,000 r.p.m. Car goes into limp mode. Cause was usually on a fairly steep mountain pass grade or freeway speed increasing to/at 3,000 r.p.m. At limp mode or an instant before, m.i.l. illumination = and the fun bucking Bronco Jag.

Checked every possible vacuum leak = none.
Cleaned MAF 3 times = still happening.
Finally decided (because it is ONLY the P0171, not coupled with a corresponding P0174, which 99% of the time indicates VACUUM LEAK) to replace the MAF.

Found one from an eBay vendor in Calif for a listing year of 2007-2008 @ $18 (yup, $18). Emailed vendor with my existing part number to assure (and c.y.a.) that it would work on my 2006. Yes, ordered, installed.

Today did the normal 150 mile lottery ticket run up over the mountain passes, gunned it on the straight-a-way freeway, lugged down on 8% grade and accelerated = NO CODE, NO HESITATION, NO LIMP MODE!!! Car also is smooth as glass on normal driving and acceleration with (what I had not really noticed over time) quicker response.

So, if you are getting ONLY a P0171 code and have first ABSOLUTELY ruled out a VACUUM LEAK, try my solution.

 

nice one, good to know

 

I would get a few MAFs from junk yard, they fit Toyota. New ones are bad pretty often.
https://www.autozone.com/engine-mana.../266826_0_4995
Click fitment button

 

UPDATE = Still purring like a good kitty. MAF solved the problem.

 

Just found the magic bullet for checking vacuum leaks. I posted a full description tonight, but spray a running engine down with MAF sensor cleaner. Every other chemical i have ever used didn't work but this basically stalled the engine.

Best of luck,
Ronks2