Per ML's request, I checked the Fan-Filter units (FFUs) in preparation for the OpLev drilling that is due to start in the LVEA this afternoon. I used the hand test and followed-up with an anemometer check on each of the four FFUs on the cleanroom. Three were functioning well: the unit at the north-east (NE) corner did not seem to be functioning. I climbed up and checked each FFU's switch to make sure it was turned to high. All of them were so I started looking for some reason why the NE unit wasn't functioning. As with computer troubleshooting, I checked physical connections first. The NE FFU's plug was not securely seated in its receptacle. Once that situation was remedied, the NE FFU started functioning properly. I also checked the dust monitor: it appeared to be taking periodic readings and the counts were good (0/0) inside the cleanroom even with all my activity in and around the cleanroom. I will follow-up with hand-held dust monitor while drilling is in progress.