M54 P0491 / P0174 tomfoolery

[luvmy03E53]

If I didn't have my Ultragauge hooked up to show me the codes every time they pop up this wouldn't be so annoying. But now I'm to the point of just being pissed off about this. Damn near every ignition cycle I get a P0491, and on about every 10th I'll also get a P0174. When the P0491 was popping up just by itself, I figured, OK, I'll just get a new O2 sensor and be done with it. But now the P0174 accompanies it and makes me wonder if I'm not gonna have to chase a vacuum leak. Chasing a vacuum leak means sending the boss away on a shopping trip so she won't just be sitting around waiting for me to finish my fun.

Would anyone be so kind as to point me in the right direction here? I'd perfer to get this resolved quickly, especially since the boss has to drive my X every couple of weeks while her F25 is in for regular inspection at the plant.

Any and all guidance / suggestions / sympathy is greatly appreciated!