A method for operating an oxygen sensor arrangement senses the oxygen in a breathing loop of a breathing apparatus. The sensor arrangement includes a primary oxygen sensor to measure the oxygen in the breathing loop, a secondary oxygen sensor to measure the oxygen in the breathing loop, and a control arrangement to obtain measures from the oxygen sensor. A test channel arrangement provides a first gas having a first fraction of oxygen from a first gas supply to the primary oxygen sensor at a position adjacent to the primary oxygen sensor. A test valve arrangement opens and closes the flow of the first gas through the test channel arrangement. The control arrangement actuates the first test valve arrangement to provide an amount of the first gas to the primary oxygen sensor via the test channel arrangement and obtain measures from the primary and secondary oxygen sensors.