The oxygen sensor is a critical component of modern vehicle emission control systems. Oxygen sensor failure can result in high vehicle emissions. Many states have inspection and maintenance (I/M) programs to detect and correct high emitting vehicles, but most programs have a repair cost limit. A study of oxygen sensor designs, prices, and applications was conducted. The study showed that many oxygen sensors are constructionally and functionally the same, but their retail prices vary greatly. The high retail price of some oxygen sensors often exceeds the I/M repair cost limit, therefore high emitting vehicles with failed oxygen sensors are not always repaired.