2003 GMC Yukon • 170,000 miles

I always find it difficult to get the total picture across when trying to ask a question so I apologize in advance. I have a 2003 yukon. I had the fuel pump replaced about 1 year ago. About 2 weeks ago the check engine light came on. The other day when I started it the fuel pump sounded weird. Then it started and it idled really low. When I pushed the gas it was almost like I wasn't. Seemed pretty gutless. It has been sluggish since. I filed up the other day and it seemed to run better, got to a half tank and running icky again. Today I tried to start it and out was on a slight hill. It wouldn't start. The fuel pump wasn't making a noise at all. So I pushed it to flat ground and it started. The fuel pump was making a knarly sound though.
The code coming up from the check engine light is P 042. Thanks so much for listening to all that! Any ideas what it could be? Fuel pump? Fuel sensor? Fuel injection?
September 1, 2011.

Check the fuel pressure to rule the fuel filter and fuel pump and also verify the OBD2 code you have-start here

Sep 1, 2011.
Thanks for the info! I really appreciate it

I'm silly! Forgot to ask the obvious next question. What if the pressure is low? What does that mean?

Could be an injector/s or pressure regulator leaking and fuel pump-

Sep 1, 2011.