You can use compressed air from the inside drain access at the plenum to force out the clog. Or you can configure a wetvac to vacuum from the outside drain access. You could also try the wetvac from the drain access at the plenum as well.OK, I'm new at this, but am trying to avoid a call to the AC guy that my husband thinks I should make. Our upstairs AC line appears to be clogged between the plenum and the drain. The water is dripping into the overflow. We ran a snake into the drain at the overflow, and that section is not clogged. Can we safely access the drain at the plenum and unclog this ourselves without damaging the plenum or drain? A quick response would be great, or I have to call for service! Thanks.