A high oxygen level in the engine exhaust indicates a lean corairfuel mixture and a low oxygen level indicates a rich mixture . The oxygen sensor, measures the oxygen content in the exhaust gas and generates a variable voltage signal . Using that feedback signal as an input, the DMEcontrol module fine tunes the air-fuel mixture.
As the oxygen sensor ages, its ability to react quickly to changing conditions deteriorates, and it mayeventually cease to produce any signal at all. To check the operation of the oxygen sensor, see 130 Fuel Injection:
Replacement of oxygen sensors at the specified intenrals ensures that the engine and emission control system wili continue to operate as designed. As the oxygen sensorages, its ability to react quickly to changing conditions deteriorates, and it may eventually cease to produce any signal at all .
As the oxygen sensor ages, its ability to react quickly to changing conditions deteriorates, and it may eventually cease to produce any signal at all. To check the operation of the oxygen sensor, see 130 Fuel Injection.

NOTE
On 1996 and later cars, OBD 11 enhanced emission standards require the engine control module (ECM) to monitor the oxygen content in the exhaust both before
and after the catalytic converter. This allows for tighter control of the tail pipe emissions and also allows the ECM to diagnose converter problems. If the DME detects that catalytic converter or oxygen sensor efficiency has degraded past a certain pre-programmed limit, it will turn on the Check Engine light, and store a diagnostic trouble code (DTC) in the ECM.

NOTE
The oxygen sensor signal is ignored until the engine reaches a specified minimum temperature . Therefore, when troubleshooting cold engine driveability problems, the oxygen sensorcan be ruled out as a possible cause.