Lately I've been trying to figure out why my fuel mileage has dropped significantly. I noticed on my ultra gauge that my upstream o2 sensors are not changing voltage like they are supposed to at idle. Instead they stay at about a constant .015 volts. When driving, however, they act normal.

I've also noticed lately that my short term fuel trims are both at -25 at idle and about -8 or so when driving. When driving the trims never reach positive numbers, they are always negative. Fuel mileage has dropped to about 9mpg. I normally (even in really cold temps) I get close to 14mpg.

Using the ultra gauge I have found out:
MAF is normal (.07g/s @ idle, .6g/s driving)
Coolant temp is normal (runs at 186*)
Air intake temp is normal
Throttle position sensor seems normal (12.5 @ idle)

One thing to note: occasionally under normal acceleration the truck bogs itself down and the fuel trims get in the double-digit positives, but goes away after a couple of seconds.

All of this has been going on for quite a while now. I am getting frustrated because I can't figure out what's causing my mpg to drop! I occasionally get at CEL that pops up, but it is only p1176 and p1178, fuel trim codes. So that leaves me with some questions:
Are the oxygen sensors bad and need to be changed?
What could be causing these symptoms?

 

Any help?