1.Disconnect the oxygen sensor connector and connect the special tool test harness (MB991658) to the connector on the oxygen sensor side.

2.Measure the resistance between terminal No. 1 and No. 2 of the oxygen sensor connector.
Standard value: 4.5 - 8.0 Ω (at 20°C)
3.If the resistance deviates from the standard value, replace the oxygen sensor.
4.Set the vehicle to the continuous idling mode.
5.Warm up the engine until the engine coolant temperature is 80°C or higher.
6.Perform a racing for 5 minutes or more with the engine speed of 2,000 r/min.
7.Connect the digital voltmeter between terminal No. 3 and No. 4.
8.While repeatedly racing the engine, measure the oxygen sensor output voltage.
Standard value:
Oxygen sensor output voltage
When racing the engine
0.6 - 1.0 V
If you make the air-fuel ratio rich by racing the engine repeatedly, a normal oxygen sensor will output a voltage of 0.6 - 1.0 V.
note If the temperature of sensing area does not reach the high temperature (of approximately 400°C or more) even though the oxygen sensor is normal, the output voltage would be possibly low in spite of the rich air-fuel ratio.
note When the vehicle is driven with high loads, the temperature of the sensing area of the oxygen sensor is sufficiently high. Thus, it is not necessary to apply the voltage to the heater.
9.If the voltage deviates from the standard value, replace the oxygen sensor.
note For removal and installation of the oxygen sensor, refer to GROUP 15 - Exhaust Pipe And Main Muffler - Removal and Installation <4B1-PHEV> .