There are a few causes of leaking in air conditioners. The most common is a clogged drain line, caused by dirt, dust or mold blocking the condensate drain. In older units, the cause could be a faulty drain pan. Aside from these two common causes, broken condensate pumps, clogged air filters and AC units with low refrigerant also generate leakage. If your air conditioner is leaking, contact an HVAC technician to diagnose the problem.