With OBD II, the Check Engine light will come on anytime emissions exceed federal limits by 50% on two consecutive trips, or there is a failure of a major emissions control system. With earlier engine control systems, the only way to uncover most emission problems is to give the vehicle an emissions test, which is not required in many rural areas. But OBD II is on every 1996 and newer car and light truck regardless of where it is registered in the U.S. And unlike an emissions test which may only be given once every year or two, OBD II is monitoring emissions performance every time the vehicle is driven. Many areas now check emissions by performing an OBD II plug-in emissions test. The test checks to see that all the OBD II system monitors have run, that the Malfunction Indicator Lamp (MIL) is working (an OFF), and there are NO stored diagnostic trouble codes (DTCs) in the PCM's memory. Most states that do emissions testing now do a quick OBD II plug-in check rather than tailpipe testing to verify emissions compliance.
This system was introduced in the early 1970s to all Chrysler Corporation cars, as well as most of those of other automakers. The EGR valve directs exhaust gas from the crossover passage into the intake manifold, and includes a temperature valve (CCEGR) on the radiator, sometimes with a delay timer and a vacuum amplifier. [Webmaster note: EGR is not normally on at idle. Early manuals on emissions control systems noted that EGR burned unused fuel, but this is not an issue in modern cars.] EGR reduces emissions and may increase gas mileage slightly; some vehicles shut off EGR during wide-open throttle in order to avoid having it interfere with power. EGR, when working, is a "good'un."
Oxygen Sensor: Oxygen sensors are part of the closed loop fuel feedback control system, associated with modern three-way catalyst emission control systems on gasoline engines. The closed loop fuel feedback control system is responsible for controlling the air/fuel ratio of the catalytic converter feed gas. During the closed loop operation, the electronic control module (ECM) keeps the air/fuel ratio adjusted to around the ideal 14.7 to 1 ratio. Signal from the oxygen sensor is used to determine the exact concentration of oxygen in the exhaust stream. From this signal, the ECM determines whether the mixture is richer or leaner than the ideal 14.7 to 1 air/fuel ratio. If the air/fuel ratio deviates from its preprogrammed swings, catalyst efficiency decreases dramatically, especially for NOx reduction. The oxygen sensor informs the ECM of needed adjustments to injector duration based on exhaust conditions. After adjustments are made, the oxygen sensor monitors the correction accuracy and informs the ECM of additional adjustments. The oxygen sensor is also an integral part of the onboard diagnostic (OBD) system which monitors the proper functioning of the emission control system of the vehicle. If the sensor detects oxygen content of the exhaust that is outside the specified range of the engine calibration, it will trigger the engine light to come on in the instrument cluster.