1. Remove the probe sensor guard to access the probe tip. 2. Unscrew and remove the old membrane cap and discard. 3. Thoroughly rinse the sensor tip with distilled or DI water. 4. Fill a new membrane cap with O2 probe solution that has been prepared according to the directions on the bottle. Be very careful not to touch the membrane surface. Lightly tap the side of the membrane cap to release bubbles that may be trapped. 5. Thread the membrane cap onto the probe. It is normal for a small amount of electrolyte to overflow. 6. Replace the probe sensor guard.