Upstream Oxygen Sensor Diagnosis (On the Road)

The purpose of the upstream oxygen sensor diagnosis is to detect a malfunction that would cause the OBD threshold for HC, CO or NOx pollutant emissions to be exceeded. It is carried out by measuring and comparing the switching periods of the upstream oxygen sensor.

I - POSSIBLE CAUSES OF FAULT

There are two possible types of upstream oxygen sensor degradation:

• mechanical degradation of the electrical component (breakage, broken wire), resulting in an electrical fault,
• chemical degradation of the component, which slows the sensor response time and therefore increases its switching period.

II - ENTRY CONDITIONS FOR DIAGNOSIS

Once the test conditions are met, the ECU averages the sensor periods recorded as PR121 "Upstream sensor average period", after removing parasitic effects, and compares this with an average OBD threshold period.

III - TEST CONDITIONS

The upstream oxygen sensor diagnosis can only be carried out after an engine operating time of approximately 15 minutes, if the preliminary conditions for switching on the ignition are met and maintained:

• no electrical fault detected,
• adaptations carried out,
• no previous oxygen sensor diagnosis performed since ignition on,
• no misfires detected,
• coolant temperature above 75 °C,
• engine speed read on diagnostic tool between 1472 rpm and 3868 rpm,
• intake manifold pressure between 292 mbar and 900 mbar.

IV - FAULT DETECTION

The diagnosis is carried out while driving, under the conditions described above, with the fuel vapour canister purge inhibited.

At the end of this test, it is essential not to switch off the ignition before reading the result on the diagnostic tool.

Any ignition switch-off will lead to incorrect interpretation of the results.

V - CONFIRMING THE REPAIR

If, after the test, the diagnostic tool has recorded an upstream oxygen sensor fault DF092: "Upstream oxygen sensor circuit", refer to the diagnostic procedure associated with this symptom.