A diagnostic/control system using a multi-level I2C bus, includes: multiple I2C bus master devices (MD0 to MDn) connected to a primary I2C bus; a I2C bus multiplexer module, which separates multiple secondary I2C buses from said primary I2C bus, and which connects as many I2C bus slave devices (SD00 to SD0N, SD10 to SD1N, . . . , SDN1 to SDNN) are needed on said secondary I2C bus; and I2C bus slave devices (SD00 to SD0N, SD10 to SD1N, SDN1 to SDNN) connected to said secondary I2C bus. The I2C bus multiplexer module provides access at any instant between any I2C bus master device (MD0 to MDn) and any I2C bus slave device on the secondary I2C bus.