- Why are you connecting SDA and SCL to pins 2 and 4?
- Why are you not supplying power to the device?
- Why are you using a LM339? That's a "Low-power quad voltage comparator", not an I2C level shifter.
- Why are you not connecting Gnd from the gauge to the LM339?
- What is the part number of the "Digital Dial Indicator Gauge"?
- What is your code?