Ok so here's the deal. My girlfriend has a 1997 v6 Malibu and it just recently had a blown intake manifold gasket in which me and my dad replaced. After getting situated with a bit of a misfire issue and a bad tps sensor we are all good. Now it runs at a rough idle, very rough. Service engine light is on so I pulled the codes and it's a P0171 (Running Lean Bank 1) which leads me to believe it's a bad o2 sensor. But which one? any help would be appreciated.