I2C sensors in ESPHome: default pins, the startup scan, pull-ups and address conflicts

Minimal bus

i2c:
  sda: GPIO21
  scl: GPIO22
  scan: true

scan is on by default, and the frequency defaults to 50 kHz. Many sensors work faster (e.g. frequency: 100kHz or 400kHz) if the wiring is short.

Step 1: read the scan

After flashing, open the logs (esphome logs device.yaml). At startup the i2c component logs every address that answered the scan.

• Nothing found: SDA/SCL swapped, wrong pins in YAML, no power or ground to the sensor, or missing pull-ups.
• Found, but the sensor component fails: wrong address: in the sensor config (many boards can be strapped to two addresses, e.g. BME280 at 0x76 or 0x77), or a different chip than the label says.

Step 2: pull-ups and wiring

• ESPHome enables the internal pull-ups (on ESP32 with the Arduino framework they're on by default, on ESP8266 always), but they're weak. For longer wires or several devices, add external pull-up resistors, or use breakout boards that have them.
• Keep I2C wires short. It's a bus meant for circuit boards, not for tens of meters of cable.

Step 3: two sensors with the same address

Either put them on separate buses:

i2c:
  - id: bus_a
    sda: GPIO21
    scl: GPIO22
  - id: bus_b
    sda: GPIO25
    scl: GPIO26

sensor:
  - platform: bme280_i2c
    i2c_id: bus_b
    address: 0x76
    temperature:
      name: "Outside temperature"

Or use a TCA9548A multiplexer, which splits one bus into several channels.

Before choosing extra pins on ESP32, check the list of pins to avoid (strapping and flash pins).