Sounds like the probe needs replacing. They sent you a new controller, but is the probe new?
Quote:
To test your PH probe do the following test:
A simple test can be performed using Windex glass cleaner with ammonia and any type of soft drink The procedure is as follows: Turn your meter or tester on and place the probe in a soft drink which is acidic and the reading will be below 3.0 then go to the Windex with ammonia which is alkaline. Your display should move very quickly up the scale to above 10.0 If the probe slowly moves up the scale then it is time to consider replacing it.
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