DDC or DDC2: Make sure the pool water level is high enough that the lid is covered (it is a water sealed lid). Check to see that there is an opening about 18 inches down on the inside of the pool in front of the canister.  The opening should be a 1 ½ inch line. Make sure there are no restrictions over or in it.  If it is not there you will need to order an O-ring, part number 005-152-4580-00. The O-ring must be silicone in place on the underside of the lid. NOTE: This is only needed if the equalizer line was not installed on the unit. If the lid has water on top and the air coming in is under the lid, then you will have to call a service tech to clear the line.  The service tech will have to temporarily seal off the side SDX2 drain and blow out the main drain line with the discharge of a pump, making it go backwards into the main drain line. Alternately he could a scuba tank to blow air backwards into the main drain line.  Note: A diver may be needed to remove the drain top, so the debris can come out. Always be sure all drains are put back into open working condition before restarting the pump in normal operation. NEVER OPERATE THE PUMP WITH A SINGLE SUCTION OUTLET DESGNED TO WORK WITH TWO SUCTION OUTLETS.