Joined: Jul 2010
Location: Gatineau, Quebec
As he pointed out to you, you need to do some troubleshooting to identify the cause of the communication errors. The diagnosis procedure will be in the service manual. Of course, the Serena was never sold in western markets, so its not like we have ready access to it, and I suspect your language of choice for it would not be English.
Maybe you could look at a service manual for an Altima or X trail that uses the same engine as yours -- models from 2002 to 2006 should use the same engine QR25DE.
You should probably examine the wires and the connections to your ECU. Did you somehow get water leaking in? You may need to find a great mechanic or to take it to a Nissan dealer. Wish I could help more, but I don't really know how. It might help if you provided more information as to when this happened and how you first noticed it, and also what year and model Serena are we talking about.