Thank you for your question,
Please make sure that you calculate the voltage provided by the fuel sensor correctly according to your vehicle's fuel tank.
An example can be found here: https://community.teltonika-gps.com/?qa=blob&qa_blobid=2421646375445313067
1. The analog output of fuel sensor should be connected to analog input of the device. As displayed in this basic connection scheme https://teltonika-gps.com/downloads/en/fmb120/FMB120-wiring-scheme-3.jpg
2. The voltage received at the device depends on what is being transmitted via fuel sensor. The device calculation of input voltage of Ain can have a 1 % error. If the interval of sensor is 0-10 V, a jump of 100 mV can occur.
3. Depending of your sensor output voltage range, device can be configured for 10 V or 30 V range, this option can be found in the "System" section. https://wiki.teltonika.lt/view/FMB640_System_settings#Analog_Input_value_range
Also you can increase the "Avg Constant" in order to decrease possible jumps.
Please note that Analog sensor's data can be impacted by vehicle moving, angle, since the fuel in a non-stacionary vehicle is moving. Check the documentation on fuel level calculation provided in the example above to solve any issues regarding determining the fuel level.
Hope this helps,
See video guide below