If you suddenly go into limp mode and find a code P0091, check your wires first before taking the car in. It's possible the wires rubbed through and are grounding out (I think the wires in question go to the fuel pressure sensor or regulator). It's also possible that maybe your fuel pump is going bad, but check the wires first. It's well worth checking the wiring harness now even if you don't have a problem yet. If you see wear in the conduit, a little preventive maintainance is a good thing.
The other morning my Cobalt SS/TC went into limp mode, it lost power, chimed, check engine light came on, and it displayed the dreaded "eng pwr reduced" message. After checking to make sure the boost tube hadn't come loose (it had not), I pulled the code and it was P0091 (fuel pressure circuit low). I searched the forum and found out this code is common when the fuel pump is going bad. So, I was about ready to take it in to the stealership for a new fuel pump when I read a thread about a wire rubbing/shorting which can cause this code. I checked the wiring harness (see photos) and sure enough, it had rubbed through and was touching metal.
A little electrical tape and I was able to make it home ok. I figured out a more permanent fix this weekend. I cut a piece of plastic (like from a gallon milk jug) and wrapped it good with electrical tape at the rub point. This gives a little non-conductive armor to the rub point. You will probably have to disconnect a connector or two and pop off the harness holder to give you more room to wrap it with tape. Gently pull the harnes towards you to work. The rub point on my car was on the smaller harness right where it comes out of the larger harness.
This first photo shows the general area where to look (driver side front of the engine near the boost tube). The red circle marks the spot. You'll need to pull the harness away from that black metal bracket to see if it has rubbed or not.
This is a closer view showing the fixed harness. You can see the outline of the plastic piece I used for armor. Tape it up real good with electrical tape and you're good to go.