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P2096 SUBARU - Post Catalyst Fuel Trim System Too Lean

Possible causes
- Low fuel pressure
- Misfire condition
- Large vacuum leak
- Air leak near sensor
- Plugged catalytic converter
- Lean air/fuel ratio
- Faulty front O2 sensor
Possible symptoms
- Engine Light ON (or Service Engine Soon Warning Light)
P2096 Subaru Description

The catalyst's efficiency is demonstrated by its ability to oxidize CO and hydrocarbon emissions. The Powertrain Control Module (PCM) compares the output signals of the front and rear oxygen sensors to determine whether the output of the rear sensor is beginning to match the output of the front oxygen sensor. Air/fuel mixture compensation keeps the frequency of the front oxygen sensor high due to the changes from rich-to-lean combusition. The catalyst causes the rear oxygen sensor to have a lower frequency. As the catalyst wears, the rear oxygen sensor’s signal trace begins to match the front oxygen sensor’s signal trace. That is because the catalyst becomes saturated with oxygen and cannot use the oxygen to convert hydrocarbon and CO into H₂O and CO₂ with the same efficiency as when it was new. A completely worn catalyst shows a 100% match between the frequency of the front and rear sensors.

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