We recently dealt with the case of a defective salt chlorinator. The unit was brand new, but was not covered by warranty because it was bough as part of a lot.
The unit was manufactured by Hayward and was branded as "Swimpure". This device is exactly the same as the Aquarite model also from Hayward.
The symptom was that the unit would power up normally, and seem to function normally, but when trying to set the level of chlorine generation using the dial knob, the LCD display would not display a stable percentage. Instead, it would display random numbers, constantly changing.
We initially called Hayward technical support in order to verify what this problem could be. Very good technical support by the way. We managed to talk to a real knowledgeable technician within a few minutes. We were told it was likely a defective circuit board that would cost a couple of hundreds dollars to change.
Fortunately, when we opened unit, before ordering the new board, we noticed that the LCD display was sitting on a small draughtboards connected to the main board through some extra long pins.
We tested this connector and it turned out that one of the pin had a bad connection.
We were able to repair the unit simply by bending the metal strip on the female part of the connector, and voila! The device started behaving properly.