Why Do Some Bosch Oxygen Sensor Threads Appear "Burnt"?

To ensure quality, Bosch factory tests each oxygen sensor in simulated exhaust gas - guaranteeing every sensor is ready to get to work! This exhaust testing may result in a "burnt" or used appearance. The testing process takes about 10 minutes, and involves a large chamber that exposes the sensors to real life conditions. Any sensor failing that test is discarded and deemed defective. This additional step in quality control means a "tried and true" product, one worthy of your ride!