How do I fix a pool salt sensor that's reading low?

Clean the sensor first — that fixes it 90% of the time. Turn off the power, pull the sensor or cell, and soak it in dilute muriatic acid or cell cleaner until fizzing stops. Rinse, reinstall, and recalibrate if possible. If that doesn’t work, check water temperature and flow; don’t add salt until you verify with a dropper test kit.