So I have had this code P0171 on my 1996 gmc jimmy for WAY to long and I thought for a long time it was a failed o2 sensor but I replaced it and it went away for awhile but now it's back again....
Most of the time it doesn't seem to do anything noticeable until I idle and it runs funny and can even die from time to time but very rarely (it will start back up tho no prob).
Any tips? Are there two o2 sensors in bank 1?
I just had a fuel injector changed at a local shop think that might have anything to do with it?

 

Low fuel pressure might be causing them to set. P0171 and P0174 are indicating a lean condition on both banks. In the early stages of fuel pump failure, fuel pressure can drop. Might want to check fuel pressure and leakdown before anything else. Key ON, engine OFF, fuel pump running, pressure must be 60psi to 66psi and it must remain above 55psi for at least 10 minutes after the pump shuts off. Post your results and we'll go from there.

EDIT: Your 96 has 4 oxygen sensors. The 2 upstream sensors are used for fuel control. The 2 downstream sensors monitor the catalytic converters and do not affect engine performance.

 

^^^+1 on Capt, they should have come from the factory with a fuel pressure gage by the jack.
One fast thing you can do is take the egr off and clean the hell out of it.
When mine gets gummed up it does the idle thing you described.

 

Check for major vacuum leaks too, I had a p0171 and p0174 and it was the result of a major vacuum leak.

 

Thanks for all the replys so far.
So as for the fuel pressure gauge I don't have one, are they relatively inexpensive?
-what is the EGR? Where can I locate it?
-also how many vacuum lines are there and where are they located as well?

-sorry for my inexperience I'm still learning

 

Most auto parts stores rent/loan fuel pressure testers for free. If you choose to purchase one, (not a bad idea when you own a Blazer), decent ones are in the $40 range.

 

Round disk between alt and AC. Thats a dead on shot its about 6" deep with two 10mm bolts. Go on google for cleaning details, 20min top's.