The oxygen sensor is the basis of the closed-loop control system on a spark-ignited Stoichiometric - engine; however,
it is also used for diagnostics. In vehicles with OBD II, a second oxygen sensor is fitted after the catalytic
converter to monitor the O2 levels. The O2 levels are monitored to see the efficiency of the burn process. The
on-board computer makes comparisons between the readings of the two sensors. The readings are taken by voltage
measurements.
If both sensors show the same output or the rear O2 is “switching”, the computer recognizes that the
catalytic converter either is not functioning or has been removed, and will operate a malfunction indicator lamp and
affect engine performance. Simple “oxygen sensor simulators” have been developed to circumvent this problem by
simulating the change across the catalytic converter with plans and pre-assembled devices available on the Internet.
Although these are not legal for on-road use, they have been used with mixed results. Similar devices apply an
offset to the sensor signals, allowing the engine to run a more fuel-economical lean burn that may, however, damage
the engine or the catalytic converter.
