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Message: <blockquote data-quote="spb_stan" data-source="post: 629738" data-attributes="member: 43545">If that is the case, it appears to be stubborn dirt and may require repeated more aggressive cleaning with a different solvent. Organic substances are often cleaned best with distilled water. Did any residue appear on the swab?  If not, and it just moved the substance around, it appears not to be soluble in alcohol so try water next, then alcohol to absorb the water with the substance in suspension. If it is waxy, water will not have any impact but mineral spirits can be effective. If organic fats, toluene or heptane are effective solvents. If it happened in the spring, being organic is the most likely contaminate. Luckily the sensor is pretty tough and chemically inert so if applied only to the sensor surface and none of he plastic it is pretty safe from chemicals.</blockquote>