Did you let your pH probe dry out (don’t worry; you’re not the only one)
Do you have a $12 pH Sensor that you bought from ebay that isn't working correctly?
Maybe that Sensor you have has seen better days.
Before you throw it away, think about trying to recondition it. Although it’s not a 100% guarranty, many pH Sensor can be given new life by reconditioning them.
All you have to do is dip the pH Sensor in this three step reconditioning chemical bath and your Sensor could be as good as new. The entire process only takes about 5 minutes.