# Catalytic Converter



## Segfault (Feb 1, 2013)

A friend of mine has 2005 Altima 2.5L.
I can hear exhaust leak somewhere under hood. He was told his catalytic converter may need replacing. I looked it up and results are confusing, there are two parts described as converters, one is directly connected to the exhaust and the other one is somewhat farther under car. 
Can somebody explain how emission control works on these cars, possible points of failure?


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## Segfault (Feb 1, 2013)

OK, found some information online. It seems this car has two catalytic converters. Does anyone know where the sensors are positioned?


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## Segfault (Feb 1, 2013)

Hmmm, I found out there are 3 sensors, I'm familiar with 2 sensor systems, anyone knows what the third one is for?


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## quadraria10 (Jul 6, 2010)

I am pretty sure we have the same engine and cat set up, and on our X trail there are 2 0xygen sensors. If you had the 6 cyl 3.5 model you would have 4. There is a front cat attached to the engine manifold, and there is a second behind the catalytic converter under the car. 
Not too sure where you read about 3 sensors, but its bad info. 
You may have a manifold gasket leak, or an actual hole somewhere. Sounds to me like your friend should bring it into a muffler shop or two and get a free diagnosis, and explanation of their findings.


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## smj999smj (Jan 1, 2006)

I've seen 4-cyl. engines with as many as 4 oxygen sensors. Bottom line is the front converter is usually the converter that is monitored by the ECM. Oxygen sensors (or, air/fuel ratio sensors in some vehicles) located before the catalytic converter are used by the ECM to help it determine if the engine is running lean or rich. It uses that information, along with other information such as TPS input, intake air temp sensor reading, etc., to adjust the amount of time the fuel injectors are open to let in more or less fuel to the engine. Oxygen sensors located after the catalytic converter being monitored are strictly used by the ECM to determine efficiency of the catalytic converter and they have no effect on engine management. The frequency of the rich/lean cycling of the front sensor should be approximately double that of the rear sensor. A ratio of 1:1 or close to that would indicate an inefficient converter and will typically result in the ECM setting a P0420 or P0430 trouble code. 
4 cylinder engines with 4 oxygen sensors will break the cylinders into two banks. For example, cylinders 1 and 3 might be bank 1 and cylinders 2 and 4 would be bank 2. Each back would be monitored by its own set of front and rear oxygen sensors.


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## Segfault (Feb 1, 2013)

Thanks! Message too short ...


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