I replaced 3 of the 4 sensors. As others have said the passenger side upstream sensor has a connector that seems impossible to get to without loosening the engine mounts and lifting it slightly. One tip I can share for the others is that both the downstream O2s would not come loose with a sensor tool. They just kept skipping over the hex 'corners' trying to round it off. I finally had to resort to using the air impact gun to persuade them loose. The key is use a sharp straight chisel bit and make an indent about an 1/8" before one of the hex 'corners' straight on, then angle the tool so that it's force causes the sensor to rotate in the loosening direction. If you don't have an air tool a cold chisel and hammer will work, just takes longer. Steve