First, we need to determine if there really is mold inside the shaft of the sensor–it might be a precipitate rather than mold. To test this, place the sensor in ~150 mL of warm water and let it sit in the warm water for 15-20 minutes. If the amount of suspected mold has decreased or disappeared altogether, then it was in fact undissolved salt that is part of the gel mixture in the sensor’s shaft. Soaking the sensor in warm water allowed the salt to redissolve. If the amount of suspected mold does not decrease, and it is still under its five-year warranty, please contact Vernier Technical Support.

What do I do about mold growth in my bottle of pH electrode storage solution?
What is the best way to clean the glass bulb of the pH electrode?