Error message E91 on the hot tub indicates a problem with the saltwater system (sanitation). The reason for the E91 error message on the whirlpool can be due to a small amount of salt in the water, a clogged electrode (follow the steps in the user manual, clean the electrode in vinegar water), an expired electrode (the electrode needs to be replaced) or a water temperature below 10°C. If necessary, heat the water and check the saltwater system for functionality. If the E91 error message appears on the hot tub, but the salt in the water is the correct amount and the electrode is clean, the electrode will probably need to be replaced with a new one. If your spa is under warranty, claim it from your dealer. If it is no longer under warranty, you can buy a new electrode from us.