I2C device not found on Arduino or ESP32: scanner and fixes

Run an I2C scanner first. It shows whether the device answers at all, and at which address. If the scanner finds nothing, the fault is wiring, power or pull-ups. If it finds a different address, your code has the wrong one.

What the message means

I2C uses two wires, SDA and SCL, plus power and ground. Every device has an address. A scanner tries each address and prints the ones that reply.

Fix it in this order

1. Upload the scanner below and open the Serial Monitor at 115200 baud.
2. If nothing is found, swap SDA and SCL. They are often crossed.
3. Check the pins. An Uno uses A4 (SDA) and A5 (SCL). The ESP32 default is GPIO 21 (SDA) and GPIO 22 (SCL).
4. Check power and ground, and check the module voltage matches the board. Do not feed 5 V logic into a 3.3 V ESP32.
5. Check the pull-ups. Many breakout boards include them. Bare chips need roughly 4.7 kohm from SDA and SCL to the supply.
6. Shorten wires and lower the bus speed if the scanner finds the device only sometimes.

An I2C scanner

#include <Wire.h>

void setup() {
  Serial.begin(115200);
  Wire.begin();
  for (byte addr = 1; addr < 127; addr++) {
    Wire.beginTransmission(addr);
    if (Wire.endTransmission() == 0) {
      Serial.print("Found 0x");
      Serial.println(addr, HEX);
    }
  }
}

void loop() {}

Why it happens

Common causes are swapped SDA and SCL wires, a missing common ground, the wrong supply voltage, a different address than the example assumes, and missing pull-up resistors. A scanner shows what is actually on the bus.

#include <Wire.h>
void setup() {
  Serial.begin(115200);
  Wire.begin();   // Uno SDA=A4 SCL=A5; many ESP32 boards SDA=21 SCL=22
  for (byte a = 1; a < 127; a++) {
    Wire.beginTransmission(a);
    if (Wire.endTransmission() == 0) { Serial.print("found 0x"); Serial.println(a, HEX); }
  }
}
void loop() {}

Example sketch: change pins and values for your own wiring. It has not been bench-tested on your hardware.

If nothing is found, check wiring and power first. Breakout boards usually include pull-ups; bare chips need about 4.7 kΩ on each line to the logic voltage.

If you see this, do this

Related

ESP32 pinout for robotics, Arduino IDE setup.

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.