I Have code U0100 and U1900 on my 2004 mazda rx8. A bunch of other codes have the same cause that is the wire harness and im not too sure what to fix or what to watch out for.

 

What's the history of the car? Is it new to you? Has it been worked on before this issue appeared?

You're looking for broken wires or connectors. Sometimes rodents eat through wiring if the car sits outside. See if you can find damage. If not that then you can try going to each connector (ECU, ABS, cluster, etc) and check the CANBUS pins for continuity between connectors according to the wiring diagram which you can google.

 

If you clear them how soon do they come back? The U codes often are random and don't recur if you clear them. They also show up with some scanners that don't behave well with the CAN bus