I think I actually have 2 different CAN bus modules that I was going to try with the Arduino but never did. This thread is renewing my interest a bit. My plan was going to be to put an Arduino in the E-box and maybe at the fuel tank to gather information that isn't available on the bus and another on the dash to run the display all linked with a RS-485 bus. I chose RS-485 partly because it seemed to very robust and good for longer distances and other projects I have and didn't know much about CAN. I have everything working (the RS485 with my own data packets and error detection) but haven't put everything together.

If you like electronic projects the DSO138 scope kit (about $15-$30) is pretty cool for the money and is more than adequate for checking most automotive stuff including CAN bus.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)