Oxidation occurs every time you fire your kiln. So just because your kiln reads 1650F doesn't mean it is firing at 1650F. This is why it is important to test your kiln (see our cone testing kit). After 75-200 firings (depending on the size of your kiln) you should replace the thermocouple. This is very easy and takes about 15 minutes. You'll need a screw driver and the replacement thermocouple. Good to keep one on hand in case your kiln goes out. Especially if you teach classes. Eventually the thermocouple will die.
This thermocouple is for the following US 110V kilns:
Xpress E12A and T or referred to as MultiMedia