Error message E91 on saltwater systems and sand filter systems with a built-in saltwater system indicates insufficient salt in the water or the need to clean the electrode or replace the electrode. If the salt in the pool water is the correct amount and the electrode(s) are clean, the electrode will probably need to be replaced. If the product is under warranty, claim it with the dealer. If it is not under warranty, you can inquire about the electrode as a spare part. Refer to the user manual for the 5-digit spare part number.