Reply To: 20×4 I2C LCD on the cleanroom pressure display only shows a row of solid blocks

Blocks on rows 1 and 3 with contrast fine means the display powered up but never got initialised, so it’s sitting in its one-line power-on mode. Almost always the I2C address. Backpacks with a PCF8574A chip are at 0x3F, not 0x27, and from the outside they look identical. Run a scanner:

#include <Wire.h>
void setup() {
  Serial.begin(115200);
  Wire.begin(21, 22);
  for (uint8_t a = 1; a < 127; a++) {
    Wire.beginTransmission(a);
    if (Wire.endTransmission() == 0) Serial.printf("found 0x%02Xn", a);
  }
}
void loop() {}

Second thing for an ESP32: the backpack on 5V has its pull-ups to 5V, so SDA/SCL idle at 5V into a 3.3V chip. It usually works and it’s not great. Either power the backpack from 3.3V (some LCDs go faint at 3.3V) or remove the backpack’s pull-up resistors and add 4.7k to 3.3V, or put a small level shifter in between. For something on a cleanroom wall for years I’d do the level shifter.