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Can someone shed some light as to what the O2 sensors do for the engine and the PCM? I know people running long tubes, usually catless ones get check engine lights. How important are the O2 sensors to overall performance and engine safety?
 

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in simple words, the o2 (oxygen) sensor measures how rich or lean your engine is, by monitoring your air fuel ratio (AFR). Usually (if not always), there are 2 sensors for every catalytic convertor the car has. If your car has 1 cat, then there are 2 sensors, if it has 2 cats, it has 4 o2 sensors.
There is one before (upstream) the cat, and another after (downstream) the cat. Direction going from headers, towards exhaust tips.

Both sensors work in relation to each other. The upstream sensor will vary a lot jumping up and down in readings, while the downstream sensor should read at a more steady flow due to the cat leveling things down. This is normal. When you remove the cat, the downstream sensor will read almost identical to the upstream one, since there is no catalytic convertor leveling things. When this happens, it will throw a code. It will not affect performance at all if the downstream sensor is bad or throwing a code, it simply means the cat isnt doing its job. This code can be tuned out from the pcm by any tuner.

However the upstream sensro is very important. If it reads incorrectly, the pcm will automatically try to adjust the AFR on its own, and can make the car run bad and loose performance.

So in other words, the first one, is important, and the second one is just for emission. Hope this helps!
 

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Thanks for that explanation. I still have one question. I have a 2003 ram 1500 4.7L. If the catalytic converter has been removed (say by the previous owner who put a different exhaust on the truck) what will that do to performance? I am getting codes from both o2 sensors and am curious what that is doing to my gas mileage/performance.
 
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