A failing pH probe will be slow to show a result and will be difficult to calibrate.
A simple test can be performed using Windex glass cleaner with ammonia, and any type of soft drink.
The procedure is as follows.
Step 1: Turn your meter or tester on
Step 2: Place the probe in a soft drink. Soft drink is acidic and should have a reading in the range of 2.7 to 3.9 pH.
Step 3: Then place the probe Windex with ammonia. Windex is alkaline and your reading should move very quickly up the scale to a point above 10.0.
If the reading slowly moves up the scale, then it is time to consider replacing the electrode the pen / tester if it does not have a replaceable probe.