If a pH probe is drifting or not holding a calibration immediately after the calibration process is performed with freshly prepared, accurate standards, then there may be a few things wrong with it.

First, consider the age of the pH Sensor. How old is my pH Sensor?
The typical lifetime for a pH sensor is about 5 years. If it is well cared for, you can expect 5-8 years of usable life. After that, you will need to replace it.

Next, inspect the globe-shaped tip of the sensor. If it is cracked or broken, then the pH sensor is irreparably damaged and needs to be replaced. We cannot repair a broken bulb. The glass bulb of my pH Sensor is broken. Can it be repaired?

Perform a quick pH reading using the pH Sensor. Use fresh buffer solutions instead of distilled water. If buffers are not available, you can use vinegar (pH around 2.5-3.5) and ammonia (pH around 10.5-11.5) as alternatives. Observe the readings and check if they change when the sensor is placed in different solutions. If the readings remain unchanged regardless of the solution, it’s possible that the sensor is defective or damaged.

Do not use water to test a pH sensor. Why doesn’t my pH Sensor read pH 7 in distilled or deionized water?

If the pH sensor is displaying a value far outside the normal range (e.g., negative or extremely high), try resetting the factory calibration and test it again using a buffer solution.

If the pH sensor consistently displays the same value despite switching between buffer solutions, it’s possible that either the electronics or the electrode bulb is malfunctioning. To diagnose the issue, isolate whether the problem lies with the amplifier or the electrode, and replace the faulty component accordingly.

Also see
Go Direct pH Sensor Troubleshooting and FAQs
pH Sensor Troubleshooting and FAQs