Check that the circuit between terminals 3 and 4 of the sensor connector (1) closed (resistance between terminals should be 74 ohms at 20°C)
If the circuit is open, replace the oxygen sensor.
Warm up the engine to the operating temperature of the coolant.
Using wires, connect pin 3 of the oxygen sensor connector to «+» battery, and pin 4 - to «-» battery terminal.
Connect a digital voltmeter to pins 1 and 2 of the sensor connector.
Periodically pressing the accelerator pedal, measure the output voltage of the oxygen sensor. When you press the accelerator pedal, the output voltage of the oxygen sensor should be 0.6-1.0 V.
Replace oxygen sensor if necessary.