The Problems of I2C - common problems and errors with using I2C in What every embedded programmer should know about ... A group blog by Microforum Staff Posted April 6 The way I2C works when you add a second device no changes should be required in your configuration. If the working address stops working it is usually either because of multiple devices with the same address or it is due to an electrical problem. Could be the capacitive loading on the bus or even reflections if you have a star with long lines without termination (could also be incorrect size of or even missing pullup resistors). What I would suggest is to try and reduce the speed after ensuring that you have different addresses. Also look at the lines with an oscilloscope and check to see if the bus is being pulled down (ACK) after the address. If not no device is recognising the address.