Rear heated oxygen sensor
To be able to diagnose the front catalytic converter, there is an oxygen sensor mounted after it in the exhaust system. This sensor is called oxygen sensor 2 or O2S 2 and is connected to control module pin 7(B) and grounded from control module pin 19(B).
In order to supply a voltage quickly after starting, the oxygen sensor must be preheated. The preheating is supplied with B+ from the fuel pump relay via fuse 3 and is grounded via control module pins 61(B) and 62(B). The preheating circuit ground is PWM so that the preheating effect can be regulated.
Preheating is activated as soon as the engine coolant temperature exceeds 50°C.
The control module estimates the exhaust temperature on the basis of engine load and speed. If the exhaust temperature is high, preheating will be disconnected to prevent damage to the sensor.
If the catalytic converter is damaged, its ability to absorb oxygen will deteriorate. The normal fluctuations of the closed loop will then be detected by the voltage from oxygen sensor 2 and a diagnostic trouble code will be generated. The diagnosis is performed once per driving cycle.
In addition to the catalytic converter diagnosis, the oxygen sensor value is also used to correct the closed loop for minor faults in oxygen sensor 1.
Optimum emission values are obtained when the voltage from oxygen sensor 2 is 0.6V.
If the voltage is 0.3V, for example, the engine is running slightly lean. The closed loop correction factor will then be held in the rich setting for a certain number of combustions before oxygen sensor 1 is allowed to affect the value again.