If one is in doubt as to whether a certain vessel is dairy or meaty or Pareve, what is the law?
Seemingly, if one desires to use the vessel for dairy or meat meals it is required to be Kashered. If one does not desire to Kasher the vessel, or it is unable to be Kashered, it may only be used for Pareve foods. If the Pareve food that is cooked in this pot does not contain a Davar Charif then it may be eaten together with dairy or meat in the same meal. If it contains a Davar Charif, it may not be eaten within a meat or dairy meal.
 The reason: As it is a case of doubt to which we rule stringently. Now, although if the pot is no longer Ben Yomo it is a case of Rabbinical doubt, nevertheless, since this doubt is due to ignorance, it is not considered a doubt at all and one cannot apply the rule of Safek Derabanan Lihakel. [See Michaber Y.D. 98/3, Shach 98/9 and Taz 98/6 regarding Daas Shotim that it is not considered a Safek even by a Rabbinical case] Furthermore, one may come to use the pot with a Davar Charif and the opposite dish, which according to many Poskim makes it Biblically considered like the taste of the pot even when not Ben Yomo. [See Panim Meiros 1/64; P”M 96 M”Z 1 based on Rashba 496; Shivas Tziyon 32; Rav Akiva Eiger, brought in Pischeiy Teshuvah 95/4]
 See Rama 95/2