Along with other T4 recruits, Frenzel reported to the Columbia Haus in late 1939, where he was first checked for political reliability and then watched a film on the supposed degeneration of handicapped people. He first worked in the laundry and as a guard at Grafeneck Castle, then he worked in construction at Bernburg Euthanasia Centre, and finally became a stoker at Hadamar Euthanasia Centre. As a stoker, he was responsible for removing the dead bodies from the gas chambers, breaking out gold teeth, and burning the bodies, as well as various other tasks around the gas chambers and crematoria.