I2C device not found: Arduino and ESP32 scanner fix guide

I2C symptoms and likely causes

I2C troubleshooting order

1. Power the module at the correct voltage for that board.
2. Connect module ground to Arduino or ESP32 ground.
3. Wire SDA to SDA and SCL to SCL. Do not follow Arduino Uno A4/A5 wiring blindly on ESP32.
4. Use a short, direct breadboard setup with one I2C module.
5. Run scanner code and note the detected address.
6. Use the detected address in the module example code.
7. Add the second I2C module only after the first module works.

I2C scanner code for Arduino and ESP32

On Arduino Uno, `Wire.begin()` uses the board default I2C pins. On ESP32, use `Wire.begin(SDA, SCL)` when you want explicit pins.

#include <Wire.h>

void setup() {
  Serial.begin(115200);

  // ESP32 common pins:
  Wire.begin(21, 22); // SDA, SCL

  // For Arduino Uno, use:
  // Wire.begin();

  Serial.println("I2C scanner started");
}

void loop() {
  byte error, address;
  int devices = 0;

  for (address = 1; address < 127; address++) {
    Wire.beginTransmission(address);
    error = Wire.endTransmission();

    if (error == 0) {
      Serial.print("I2C device found at 0x");
      if (address < 16) Serial.print("0");
      Serial.println(address, HEX);
      devices++;
    }
  }

  if (devices == 0) Serial.println("No I2C devices found");
  delay(3000);
}

Arduino and ESP32 I2C pins

Pullup resistors and voltage level problems

I2C uses open-drain signals, so SDA and SCL need pullup resistors. Many breakout boards include pullups, but bare sensors or custom PCBs may not. Too many breakout boards can also create overly strong pullups.

• For ESP32, pull SDA/SCL to 3.3V unless the module and wiring are level-shifted safely.
• For Arduino Uno 5V projects, check whether the I2C module tolerates 5V logic.
• Typical pullups are often in the 4.7k to 10k range for simple short-wire projects.
• If many I2C modules are connected, too many onboard pullups in parallel can affect bus behavior.

Common I2C addresses

Example code often assumes one address, but modules can ship with a different address. Scanner output wins over assumptions.

When multiple I2C modules fail together

If each module works alone but the combined bus fails, you may have an address conflict, one faulty module holding SDA/SCL low, long wiring, weak power, or too much bus capacitance.

• Scan after adding each module.
• Check whether two modules share the same fixed address.
• Keep wires short for breadboard projects.
• Avoid mixing 3.3V-only modules with 5V pullups.
• Disconnect the last added module if the scanner suddenly finds nothing.

FAQ