#4 – The Drain Pan is Damaged
The overflow drain pan is made of either plastic or metal, but in either case, it can corrode/deteriorate over time. If you know where the drain pan is located, you can shine a flashlight on the area and check for damage.
If it is damaged, it just needs a very simple replacement.
#5 – The Drain Lines are Clogged
If a clogged condensate drain line is the culprit, you will probably notice a lot of flooding underneath your unit. It can potentially be dangerous if your air conditioner is stacked on top of your furnace, which can cause the unit below to rust.
If the drain lines are clogged, a special vacuum can be used to clear the way.