3DPiper:
You are also correct in that the toner chip comes pre-programmed with a number that the printer deducts from (and writes back to the chip) until it is at zero.
Is it writing back to the chip or is the chip just a serial number that the printer sees a new toner cartridge and then assumes the toner is full and subtracts the usage level internal to the printer and not writing to the chip.
Until you can dump the contents and see if it changes between prints you cannot be sure.All of that is documented very well on the link I posted above.
But the link you supplied seems to be for a totally different printer and toner chip so may not be relevant to the one your using.
The ultimate way to do this would be to use a logic analyser to capture the data traffic between the printer and the toner chip to get an idea of the wire commands used.
Does the chip use 3.3V or 5V and what voltage is your arduino working at?