583 Onboard Monitoring Of The Catalytic Converter

The USA OBDII and impending European legislation requires that vehicle emissions systems are equipped with the facilities to illuminate a warning lamp (malfunction indicator lamp or MIL) should the catalytic converter cease to function correctly. In order to meet this requirement it is current practice to fit a second oxygen sensor downstream of the catalyst, as shown in Fig. 5.34.

Exhaust flange to engine

Exhaust flange to engine

In Fig. 5.34, A represents the upstream oxygen which is on the engine side of the catalyst. It is this sensor that provides the feedback signal that the ECM uses to control the air-fuel ratio within the required limits. The second sensor at B sends a signal to the ECM that is used to determine the efficiency of the catalyst. The voltage amplitude of this second sensor signal is the key to assessing the catalyst efficiency. As the catalyst ages, or is damaged by incorrect fuel etc., the voltage amplitude of this second sensor increases.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

Don't pay hundreds of dollars to find out what is wrong with your car. This book is dedicated to helping the do it yourself home and independent technician understand and use OBD-II technology to diagnose and repair their own vehicles.

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