What board would cause Neg pressure test to fail

If it is a board issue, then it could be the sensor, actuator or function board. The Dialysate pressure is transmitted through all 3. At least according to the electrical block diagram. But that would be a last case scenario for me to start swapping those around. I would check for air (induced negative pressure test), leaks (positive pressure test, or rinse) Check de-air and flow motor pressures. I always start off by running the test first and watch the pressure to see what it's doing during the test. Does it slowly leak out? Does it not get up to pressure limit after multiple minutes? Fill program happens in the middle? Sometimes seeing what it's doing can help narrow down where to focus on.