Ensure the unit is in Sonar Mode. Increase the sensitivity to view a weaker sonar reflection from the thermocline. The temperature difference has to be great enough and changed quickly enough vertically to display as a thermocline or it will not reflect the sonar signals. Since the density of the water changes with it's temperature, the sonar signal will reflect off the change in water density. The change in water density creates thermocline. Examples below show what thermocline might look like.