Lambda

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Oxygen sensor (N52)

Upstream (Before Cat) oxygen sensors is used to regulate the AFR by sensing the amount of exhaust oxygen. while the downstream (After cat) oxygen sensors are used by the PCM to measure catalytic conversion effeciency. Below are healthy values at idle for a N52 engine.

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# Before Catalytic
Front
Upstream		 After Catalytic
Rear
Downstream		 Lambda
Bank 1 rich - (2.00V ~ 2.04V) - lean lean - (0.65V ~ 0.75V) - rich rich - (1.00) - lean
Bank 2 rich - (2.00V ~ 2.04V) - lean lean - (0.65V ~ 0.75V) - rich rich - (1.00) - lean
A lambda value less than 1.0 means the engine is running rich, while a value greater than 1.00 implies a lean mixture.
Pre-catalytic voltage lower than 2.0 Volts indicate a rich condition and/or a partially clogged catalytic converter.
Post-catalytic voltage higher than 0.75 Volts indicate a rich condition and/or a partially clogged catalytic converter.
Partially catalytic converters usually drive fuel/air mixture multiplicative adaptations negatively down to less than -5% on each bank.
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