O2 sensor pre cat
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Racer
Joined: Oct 2015
Posts: 1,810
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From: Colorado
O2 sensor pre cat
Because of some recent threads, I felt the need to make this post. The pre catalytic O2 is what the ECU uses to determine how lean or rich the engine is running. The O2 sensor usually goes bad gradually like dampers tend to do. The sensor begins reading slower and slower. The engine goes very quickly from lean to rich continually as it runs. The ECU has to adjust the amount of fuel very rapidly. The only way it can do that effectively is if the O2 sensor is reacting fast enough. ( usually) Long before a sensor fails it start reacting to slow to do this optimally perform this function. ( I say usually because things can happen to it that cause it not to function at all). I see the pre cat O2 as a wear item that needs replacement just like a spark plug. This slower reaction of the seniors over long periods can create new problems. One obvious is loss of fuel economy. Engine speed fluctuations can happen. Over long times ruin catalytic converters prematurely. If you look at live reading of the pre cat O2. It should move rapidly from lean to rich. I'm attaching a screenshot of this. I can see from this that mine is going to get replaced soon. Probably in another five thousand miles or so I will.
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