It's highly unlikely that the probe is malfuntioning since when I use to change the batteries before I pruchased the AC
Adapter, I use to happen to me, what I use to do is keep it on for at least a day and it adjusted itself. If you want to make sure for comfort reasons, I would test the PH
and then wait a day or two, if that doesn't work then yes your probe may be defective. Do you have your probe located inside your tank where fish can pick on it or in your sump. The reason why I ask is since the time I had the probe in my tank and my triggers and tangs were picking on it, the probe had to constantly be replaced and caliberated. After it was placed in my sump, I only caliberated it once every three month's, replace the probe every 1 1/2 Year and test the PH
with a differenct test kit monthly to compare results.