[RedFRS4me]

Originally Posted by Wepeel

There must be some logic to determine if the primary AFR or O2 sensor are bad, because if one is bad then your AFR readings are unreliable, and the car has no idea what is going on from a fueling/combustion standpoint. This is just a wild guess, but in steady state conditions like cruise, the ECU uses this time to do comparisons and make sure both are reporting somewhat similar AFR. There might be some kind of aggregate error that builds and as it builds the car targets richer and richer to make the car more safe, and one it crosses a certain threshold the car decides it's safest to just go open loop rich.

The healthier both sensors are, the lower the likelihood this will happen. And if a sensor is degrading, and continues to degrade, open loop will happen faster and faster. I suspect if a car has this problem that it will happen faster as the sensor degrades more.

^That's all a guess. But when I put an older, more questionable O2 in my car it didn't take long at all to hit open loop. My original bad sensor took about an hour at 70+ to hit open loop - this other bad one took 10 minutes at 55-60 to hit open loop, and the brand new sensor did a 2hr+ 70 mph cruise with perfect AFR.

The other thing I noticed with the OFH is that the AFR sensor is located to pick up readings off of only one bank, and the O2 is sensing after the full merge. That may have something to do with it?

I curious as well about the second O2 location. But for now I'm driving on just the AF3 tables zeroed and the problem has gone away with no seconday O2 feedback interrupting my cruising AFR.