I think I used top ones from NHL and felt a difference with performance. We worth the expense. O² sensors are along exhaust, should be 2 on each side, passenger and driver before catalytic converter. Weird in my case, I had a 2003 2500 5.7 but only had 3 O² sensors, mechanic theorized that the truck, one of first built in 2003 had been adjusted for a 5.9 gas engine exhaust system that was probably left over from the 2002 2500 gassers. Those had 3, 5.7s were designed for 4. And one more thing, when replacing one of the wires of the sensor became compromised so even with new sensor the CEL was still on. Had to find replacement connectors and that fixed issue. Be gentle with the wires.