Simplified Test Procedure (from bosch)
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.
