I recently worked on a 737-400 with a IRU data bus output failure.
In this case, there were about 10 user systems including Auto-Pilot, Auto-Throttles, TCAS, Ground Proximity, Weather Radar, and others.
The issue that I was seeing was a grounded "blue" (or low) data bus wire. It is somewhat confusing, but "low" does not mean ground when dealing with data bus signal wires,
With either the red (high) or blue wire grounded, the data from the transmitting unit will not be seen by the users. I have also seen in the past where a user system component will have a shorted or bad 429 receiver circuit which takes the whole bus down.
With this particular aircraft's problem, I pulled or unplugged every user component and the IRU itself. I wanted to make sure that the ground I was seeing was not being read through one of the boxes. The ground was still present with all boxes removed. Due to weather conditions and allotted time, I was not able to proceed further with the troubleshooting.
The actual problem turned out to be a bent pin on a rack connector that shorted the blue signal wire to shield ground. I was not present for the actual find. The technicians working on the problem had to remove rack panels and open up quite a few areas to find this particular plug.
As a note: With everything installed on a working system, I read higher than 2 Meg-ohms to ground on both the blue and red wires.