Working on my 2010 Fiat Ducato 250 series. I have a parasitic draw of around 200ma. There are two codes which won't go away, U1602 CAN Line, Bus Off Intermittent in the Marelli Instrument Cluster, and D602 CAN Line Error, Bus Off in the Body Computer. These two codes come back immediately after clearing DTCs. I've also seen various other DTCs from other modules relating to failures to communicate with the Instrument Panel / Cluster, but those are rare and don't come back after being cleared.

So I had a theory that my parasitic draw might be due to a communication error on the CAN bus not allowing all modules to go to sleep. So I hooked up my PicoScope and took a quick look via the B CAN pins, 6 and 14, on the OBDII connector. My first observation was that the voltages weren't what I expected. I've read that B CAN high and low should switch between 2.5 - 3.5v and 2.5 to 1.5 volts respectively. I saw signals switching from 5v to 1v and 0v to 4v. However the signals looked well formed and the two were always the inverse of each other so I'm guessing maybe that's just a non-standard Fiat thing? Anyways, when I switch the engine off, the signals at the OBDII connector continue until I engage the alarm system and then they stop. As a note there is a third party immobilizer system installed.

Here is the layout of the network ( NBC = Body Computer, NQS = Instrument Panel, NCV = Node Convergence ):

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As can be seen, all the modules on the B CAN bus are connected to the Body Computer via a gateway and the EOBD socket is not necessarily physically connected to the bus.

So I guess there are a few questions:

  1. Does the EOBD socket see everything transmitted on the bus or is there mediation by the gateway?
  2. If there is mediation by the gateway, is it possible there is activity on the bus that I'm not seeing
  3. Does a lack of activity on the B CAN bus imply that all modules connected to the bus are asleep?
  4. Why am I seeing non-standard voltages at the EOBD socket?

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