How to tell if an AC drain is blocked?

You can determine if your air conditioner condensate drain is blocked by looking for several physical and operational warning signs around your indoor air handler and outdoor drainage outlet. First, check the indoor drain pan located beneath your air handler; if you see standing water, sludge, or a floating safety switch, the line is likely clogged. Second, go outside to where your PVC drain pipe exits the exterior wall of your house. If the air conditioner is running or has been running recently, but no water is dripping from the pipe, a blockage is trapping the condensation indoors. Third, if your air conditioner suddenly stops blowing cold air entirely while the indoor fan continues to run or the system refuses to turn on at all, a safety float switch may have tripped because water is backing up from a clogged drain line. Finally, a musty or damp mildew odor near your indoor vents is another strong indicator of stagnant water trapped in the system.