As modern OBD II technology marches on, it's clear that the zirconia-based oxygen sensor now is being sold more as a basic repair part than as a preventive maintenance part, and it changes the way we diagnose and sell oxygen sensors. To illustrate the difference, let's remember that an oxygen sensor replacement used to be considered part of an engine "tune up" that may have involved periodically adjusting air/fuel mixtures, ignition timing, idle speeds, valve lash and replacing fouled spark plugs. Today, most shops replace spark plugs and filters as part of scheduled maintenance and, when a malfunction indicator engine light (MIL) complaint does arise, they use scan tool-based technology to determine if an oxygen sensor is the cause.

Of course, let's keep in mind that, during the early 1980s, oxygen sensors quickly became contaminated with fuel-borne dirt and ethyl lead gasoline, not to mention oil ash from oil-consuming engines. In most cases, the OBD I Check Engine light wouldn't illuminate until the oxygen sensor became marginal in performance.

Today, fuel is refined to higher standards and engines run much cleaner, which means that an oxygen sensor replacement is needed only when an OBD II system detects degradation in oxygen sensor performance. When degradation is detected, the PCM turns on the MIL to warn the driver and to indicate to the technician that an oxygen sensor-related diagnostic trouble code has been ...