Error 303 (and sometimes error 802) is an indication you scale’s load beam has experienced a weight beyond the Pearl and Lunar’s capacity of 2000g (~4.4 lbs), a physical shock such as a portafilter or tamper dropping on the scale, or damage from dropping the scale. It can take weighing an object over the scale's capacity just once to damage the load sensor.
You can return the scale to functioning condition by performing a linear calibration, with the caveat that the upper reaches of the weighing range (1500g - 2000g) might be inaccurate, depending on the damage experienced. To do this, you will need three 500g F2-class calibration weights. You can find these at in our online store here
If you are unable to perform a linear calibration, or a linear calibration does not correct the issue, please contact email@example.com to have your scale serviced.