1999 1500 5.9L 160k miles. My beast has been running fine until a few weeks ago. I started noticing a minor misfire at idle and at speed. Tested and found no vacuum leaks but the problem kept getting worse, so I figured the old girl was due a tune up ... cap, rotor, plugs and wires. Now it is even worse. No check engine light. Got out the scanner and along with a P0171, it showed every single cylinder misfiring. Started looking at O2 voltage and short and long fuel trims. In closed loop, upstream O2 is not oscillating siting low but it does change with rpm changes, so I was suspecting that might be the issue. Downstream O2 pretty much echoes the upstream .. so now I'm wondering also a possible cat issue? But I looked and short term trim is maxed out low and long term trim is maxed out high...is that possible?!? Maybe the ECM has lost it's mind? Any help would be wonderful.