There is an issue with some models that forces the controller back to the Large Dose state on the 00 setting after power up.

To fix this, switch the unit to Ion Action and then press the ‘+’ button.

You can verify that the issue is fixed by unplugging it from the power source and then plugging it back in. The unit will now power up in Ion Action mode with the setting you chose. You can now set the Ion Action to the appropriate level for your pool or spa (details in the operating manual).