Below is a clip from a Bosch Product Information sheet. Though you were driving instead of at a steady 2000 RPM, it appears that your O2 sensor failed your test since it dropped to 0 V.

Since that is the lean side, and because a lean burn can mean higher combustion temps, and because higher temps favor NOx formation, that could be your cause. Also, depending upon the sensor, the Bosch application catalog recommends a replacement interval of 60k or 100k miles.

I can’t quite make sense of the last line of the clip regarding the constant voltage, but there it is in case it makes sense to someone else.

Charley

Quote:

Simplified Test Procedure
Locate the oxygen sensor and determine the wiring layout. On heated sensors check for heater element continuity, adequate 12 volt supply to heater and correct ground when the engine is running.

Note - Many late model vehicles have the ground circuit of the oxygen sensor heating element controlled by the engine management ECU for sensor temperature control purposes. Do not supply direct voltage or external ground to these circuits.

With engine speed at ~ 2000 RPM the sensor voltage should be seen to cycle smoothly between 100 – 900 mV (0.1 – 0.9 Volts) approximately eight times or more every ten seconds as shown.

A contaminated or tired sensor will be slow to cycle between the sensor voltage limits and may not generate the full potential of 900 mV. It may also be noticed that the lean swing may drop to zero voltage.

Remember!! – The oxygen sensor is a battery, a sensor that generates 900 mV constantly is generally not faulty. Faulty sensors, like a failing battery, will be slow to cycle between the control limits and be generally slower to react to mixture changes.