Funny, (well not really) I bought electrode storage solution and electrode cleaning solution with the meters, but haven't used either yet. You might try cleaning with a good part A and B solution. You dip it in A, then B, then A again, for 5 minutes each, then rinse well. Or, you might consider storing it in neutral calibration fluid (7) for a while, and see if that'll bring it around. Then 4 probably would stop working.