LCD 16x2 not displaying: Arduino I2C LCD fix guide

Symptoms and likely causes

Adjust contrast first

On many I2C LCD backpacks, the tiny blue potentiometer controls contrast. Turn it slowly with a screwdriver. If contrast is too low, the display looks blank. If contrast is too high, you may see black blocks.

Find the real I2C address

Many tutorials assume address 0x27, but some LCD backpacks use 0x3F. Run an I2C scanner before changing libraries or rewiring the whole project. On Arduino Uno, SDA is A4 and SCL is A5. On ESP32, you may need to set pins explicitly with `Wire.begin(SDA, SCL)`.

Minimal LCD test code

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("PCBVault");
  lcd.setCursor(0, 1);
  lcd.print("LCD test");
}

void loop() {
}

If your scanner reports 0x3F, replace 0x27 with 0x3F. Test this alone before adding sensors or menu code.