ISS and OSS sensors are located on the Jeff side of 4LB0-E and 4L85-E automatic transmissions; the ISS sensor near the front of the transmission and the OSS near the rear. ISS and OSS sensors are not used on 4L60-E and 4L65-E automatics, which are equipped instead with a single Vehicle Speed Sensor (VSS). Disconnect the cable from the negative battery terminal. Raise the vehicle and support it securely on jackstands. The ISS and OSS sensors are identical in appearance (and are replaced exactly the same way), so make sure that you've correctly identified the sensor that you wish to replace. Disconnect the electrical connector from the ISS or OSS sensor. To remove an ISS or OSS sensor, disconnect the electrical connector and remove the sensor mounting bolt. Place a drain pan underneath the sensor that you're going to replace. Remove the sensor mounting bolt and pull out the sensor. Installation is the reverse of removal. When you're done, check the transmission fluid level and add fluid as necessary.