First, test the pH sensor in a standard buffer solution of known pH. It should give a stable reading in a buffer solution. Many users try to test a pH sensor in water not realizing there are not enough ions present to get a stable reading.

Next, try calibrating your pH Sensor. For the most accurate measurements with this sensor, we recommend calibration. It is a simple process that takes only a few minutes, see How do I calibrate my sensor?

In order to calibrate a pH Sensor, or to confirm that a saved pH calibration is accurate, you should have a supply of pH buffer solutions that cover the range of the pH values you will be measuring. For more information about pH buffer solutions, including recipes for preparation, see Where do I get pH buffer solutions?

The accuracy and response times of pH Sensors change as they age. While a well-maintained pH Sensor will work well for many years, it will need to be calibrated more frequently as it gets older.

If your pH Sensor is very old (5+ years old), it may need to be calibrated before each use. At some point the probe will no longer read accurately, whether or not it has been calibrated. How old is my pH Sensor?

If your sensor was stored dry, see The pH electrode I have been using no longer seems to work or is slow to respond. Is there anything I can do to fix it?

Here are some additional points to consider if you are troubleshooting a pH-BTA (cabled pH sensor):
1. An important indicator of the viability of a pH Sensor is the difference in potential (delta V) between two buffer solutions when calibrating the probe. For example, we use pH 4 and pH 7 buffer solutions to calibrate pH Sensors. The potential reading in a pH 4 buffer solution should be very close to 2.50 V. The potential reading in a pH 7 buffer solution should be very close to 1.75 V. More importantly, the delta V between pH 4 and pH 7 should be 0.75 V.

2. As a pH Sensor ages, the delta V during calibration lessens. A general (but not hard and fast) rule is that when a calibration using pH 4 and pH 7 solutions produces a delta V of less than 0.5 V, the accuracy of the probe will be poor and probably only suitable for strong acid-strong base titrations.

Here are some additional points to consider if you are troubleshooting a Go Direct pH sensor:
1. The BNC connection point is sensitive to touch. It’s important to not hold this part of the sensor during data measurement. You get more stable readings when you stand the pH sensor in an electrode support or clamp.

2. An important indicator of the viability of a pH Sensor is the difference in potential (delta mV) between two buffer solutions when calibrating the probe. For example, we use pH 4 and pH 7 buffer solutions to calibrate pH Sensors. mV readings in various buffers according to Nernst equation:
pH 4 = 177 mV (0.1 pH unit is about 10 mV so within specification is between 167 mV and 187 mV)
pH 7 = 0 mV (-10 mV to 10 mV)
pH 10 = -177 mV (-167 mV to -187 mV)

RELATED:
How do I save a custom calibration to my pH sensor?
The pH electrode I have been using no longer seems to work or is slow to respond. Is there anything I can do to fix it?
pH Sensor Troubleshooting and FAQs
Go Direct pH Sensor Troubleshooting and FAQs