When it can't reach a ph of 10 or 4 or when it has a hard time stabilizing is usually when they are going bad. About a year give or take depending on how well the probe was taken care of. I think that probe you have is bad.
I've had good luck with the pinpoint and heard they last the longest for the price. You can get one for less than $50. They need to be cleaned at least monthly, and should be calibrated often as well. I usually calibrate mine once a month. Not so much when the probe is new.
Stevie D
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