Apparatus and method for determining the identity and concentration of a component in a test atmosphere having a known concentration of oxygen. At least one solid electrolyte oxygen sensor (10) is used, with each sensor having a solid electrolyte wall in contact with, and interposed between, a first electrode (11) and a second electrode (12). The first electrode (11) is in communi­cation with the test atmosphere. A first negative voltage (21) is applied across the first and second electrodes of a sensor, causing electrochemical oxygen pumping and a first electric current to flow through the corresponding solid electrolyte wall. The first negative voltage is selected to cause the first electric current magnitude to be on a first current plateau. The magnitude of the first electric current is measured. A second negative voltage (22) is applied across the first and second electrodes of a sensor, causing electro­chemical oxygen pumping and a second electrical current to flow through the corresponding solid electrolyte wall. The second negative voltage is selected to cause the second electric current magnitude to be on a second current plateau. The magnitude of said second electric current is measured. A table (23) correlating pairs of numbers repre­senting various combinations of first and second electric current with corresponding identity and concentration of one or more components is provided. The measured first and second magnitudes of electric current are matched with the pairs of numbers in the table and the corresponding identity and concentration of one or more component is determined.