According to Jared Barry, industry professional and bedding specialist, there are are several methods to determine the problem that is affecting your Select Comfort Bed. A common issue is one side deflating more quickly than the other. Each side is connected by separate tubing to an air pump.
By switching which hose is connected to which side of the bed, you will be able to rule out whether the issue is with the air pump. If the opposite side deflates, than likely the issue is the air pump. If it does not deflate, but the side with the initial problem does, then you will need to move onto the next test.
To further test the deflating side, remove the air tube from the mattress and put the end cap on. If the side still deflates you know that the problem is with the chamber or with the O-ring.
For any of the above problems, you will need to contact the company for repairs; they typically cannot be corrected by an amateur. Be sure when performing these tests to exercise patience, as a slow leak can take many hours to become apparent.