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.

 

Good to know, Coleroad. Thanks for the info.

 

Can I get these type of screens from my OB reader

 

What kind of obd reader?

 

The screenshots I posted today are from my iPad. I was using my elm-327 dongle with obd auto doctor pro.

 

I have an action ob that I bought last summer
Theazz

 

An Actron obd? If so what model? I see some of their readers will graph.