Oxygen sensors functional check: Preconditions

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Oxygen sensor operating readiness
The oxygen sensors can only emit a valid signal when at operating temperature and when the engine is running (after reaching the end of the dew point temperature). The oxygen sensor upstream of the oxidation catalytic converter reaches operating readiness when coolant temperature is between 70 °C and 80 °C.

The oxygen sensor downstream of the catalytic converter needs a minutes more than the upstream oxygen sensor to achieve operating readiness.

NOTE: Even if the engine is already at operating temperature when it is started, it still takes a few minutes for the oxygen sensors to become operational.
Stored faults
If electrical faults are stored for the oxygen sensors, operating readiness is not granted and the functional check is blocked. In this case the electrical faults must be remedied first.