I2C between 2 or more Arduino boards HELLO I have already connected with I2C 2 Arduino's all is working with comunication on Bus I2C. My Setup is :
MASTER with LCD I2C , RTC1037
SLAVE some sensor not I2C

I allready put 2 pullup resistor 10Kohm on SDA and SCL

I need to switch off the slave arduino but when i switch off the Slave board the Master Arduino stop working.Why? It's possible to continue working with master board and the slave board switch off? Thank's for help
You need to isolate the slave device from the I2C bus since it is causing interference with the signals.

Adafruit have published a good application note about level shifting in I2C busses using MOSFETs, and it also contains a bit about bus isolation:


  2.3.4 Isolation of the powered-down “Lower voltage” section.
  
  An additional feature of the level shifter circuit in figure 2 is the isolation of the “Lower voltage” section when
  VDD1 is switched off. In that case VDD1 is about 0 Volt and the MOS-FET’s [sic] are switched off because VGS is
  below the threshold voltage. The “Higher voltage” section is not hindered and stays operational. To assure a
  noise margin, the MOS-FET’s should have a minimum threshold voltage VGS(th) of e.g. 0.4V and VDD1 must
  stay below this value. The isolation feature can also be applied if no level shifting is required, VDD1 and VDD2
  may have the same value, e.g. both 3.3V or both 5V.