As has been suggested already, your MAF might be dirty. Cleaning it is very cheap and easy, and has been known, in many cases, to clear up P0172/P0174 issues. It might not solve your problem, but it's so easy,and so cheap, that it makes very good sense to try it before you even think of lookng for a more expensive cause.
It's very easy. If you don't know what your MAF is, just look at the large hose that runs from the air filter to the throttle body. The MAF is the thing that connects the two sections of that hose. It almost looks as if it's primary function is to connect the two shorter lengths of hose to make a longer hose.
It's very easy to remove the MAF, and later, to reinstall it.
One you have it out, here's what you do to clean it:
Look into the side of the MAF that goes toward the air filter. You'll see two small wire coils, close together. That's your target. Spray these liberally with standard aerosol carburetor cleaner; and then afterward, spray it liberally with non-residue brake cleaner.
Reinstall the MAF (making sure you get the airflow direction right),
reset your PCM, and give it a try.