why does my mileage say 'no bu5'?

The instrument cluster is now an electronic computer that talks back and forth to the other computers over the "data buss", just like in your home computer. When you get this message, it means the cluster is not receiving information from the Body Computer, Engine Computer, or Transmission Computer. If other things controlled by the Body Computer don't work, like the power locks or interior lights, suspect that computer. It's much more common to have just a cluster failure. One common cause is a loose pin on the connector. Those can be resoldered on some vehicles. A clue to that is the gauges will work intermittently.