Unstable analog sensor readout I have the Arduino Uno (non-original replica) board, general SPI lcd, and GUVA-S12SD analog UV sensor module.
The board (powered via USB) reads the data from the analog pin 0, converts it to voltage (multiplying the readout by (5.0/1023.0) ) and prints out to the LCD.

Being illuminated with an UV-flashlight, the sensor gives unstable readouts varying for around 0.03-0.04 V each readout cycle. I am a complete noob in electronics, so I want to ask how can I check where the problem is? Is this my error in wiring, faulty module, just normal behaviour of an analog sensor or something else?

Mine sensor module isn't from Adafriut and looks a bit different, the one in the diagram is just to show the general wiring.

Thanks! 

(Sensor module description: https://www.electroschematics.com/11509/guva-s12sd-uv-sensor-module-circuit/ )

The code of the sketch is:

#include &lt;Wire.h&gt; 
#include &lt;LiquidCrystal_I2C.h&gt;
int sensorValue;
float sensorVoltage;
LiquidCrystal_I2C lcd(0x27,16,2);  // display setup
void setup() {
  lcd.init();                     
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("");
}

void loop() {
  sensorValue = analogRead(A0);
  sensorVoltage = sensorValue * (5.0 / 1023.0);
  lcd.setCursor(0, 0);
  lcd.print(sensorVoltage/0.1); // rather low voltage is expected
  delay(1000);
}


Update: I tried to measure the output with a multimeter (as Vasekdvor suggested in answers) and it showed very similar results: continuous 0.04-0.07 V oscillations. Their magnitude seems to grow along with the intensivity of the illumination.

With sensor being dark, multimeter shows 0.006 V (changing to 0.007 V occasionally) between A0 pin and the ground.
the sensor gives unstable readouts varying for around 0.03-0.04 V each
  readout cycle


If your sensor outputs 0 to 5V and you're only getting 0.03 to 0.04 variance then I would say that it's working pretty well.  You can get a more stable output by averaging several readings before printing them.  

Additionally, for even more stable readings consider powering your Audrino from a battery.  USB power can induce a considerable amount of noise if it's coming from a PC or Laptop USB port.  Also, there is a low noise mode where you can put the Arduino to sleep during your ADC reading (and it wakes up afterwards) which helps a lot with ADC noise reduction.