This is a picture of the lead and connector for the front downstream O2 Sensor. The lead curves up from the sensor to the connector. There is a little tab on the connector that must be lifted to separate the two halves of the connector. Practice separating the connector that is visible at the front of the engine on top, the front upstream sensor. Then you’ll be able to separate this one easily. The rear sensor is similar but more difficult to get to. You can get two hands onto the front sensor but you’ll be able to get only one hand on the rear one. If you stick a tiny screwdriver under the tab of the rear one, it will make it easier to separate. Next, unbolt the front and rear cat-to-Y-pipe connections.