When the pH probe is new and has been in storage for a period of time it may need conditioning before initial calibration can be successfully completed.

To do this stand the probe in some saltwater from your aquarium for 24-48 hours so that the bottom 30-40mm is covered before performing the calibration steps. Never leave a pH probe standing in RO or pure water for any length of time as it will damage the probe.

If the probe has been stored somewhere cold it is possible for some of the KCL solution inside its body to form crystals. In most cases this will not be a problem and they will naturally dissolve over time, however if the new probe will not calibrate and these crystals can be observed it will be necessary break them up. Stand the probe in warm to hot but not boiling tap water (do not use RO water as it will damage the probe) so that most of the body is submerged for 30 minutes, the probe must not be submerged completely, the black top section should always be clear of the water. After this shake the probe like you would a glass thermometer to help break up and dissolve some of the crystals. Then repeat the normal calibration process.