No. The sole function of the rear O2 sensor is for measuring the effectiveness of the cat converters. The front O2 sensor tells the ECU everything it needs to tune the running engine. If you swapped out your cat converters with straight pipes, as some do, the car runs just fine. But the ECL code will say that your cat is bad.
I was not aware of that. I thought that the mapping was the result of both sensors. So the ecu makes no adjustments to mixture as a result of cat efficiency?
I don't think that is correct at all. The rear sensor is used to correct for drift in the front sensor switch point. This is vaguely mentioned in the WS manual on page C16; This keeps the short term fuel trim close to stoichiometric. Here is a more complete explanation of the operation. The rear oxygen sensor, located after the catalyst, is used for fuel trim corrections on some OBD-II vehicles. By virtue of its location, the rear sensor is generally protected from high temperatures and much of the contamination that affects the front oxygen sensors. In addition, the rear sensor sees exhaust gases that are equilibrated – they have already been converted by the catalyst so that there is very little residual oxygen. This allows the rear sensor to respond to much smaller changes in exhaust gas oxygen content. In turn, it then possible for the rear sensor voltage to remain near the 0.45 volt switchpoint. This characteristic allows the rear sensor to be used for fuel control. Under steady rpm and load conditions, the short term fuel trim bias can be adjusted so that the rear sensor voltage is maintained near the 0.45 volt switchpoint. This ensures that the catalyst is getting a stoichiometric exhaust gas mixture, despite any shift in the front sensor switchpoint.
I don't see that in the F355 manual. I only see the vaguely mentioning of the O2 sensor being used "in case" the front one is aging. Implying then if the front one is not aging, the rear is used only to judge cat efficiency.
Like I said, it is vague in the WSM. If we all had a dime for everything that was not fully explained in the WSM we would all be very wealthy. The second quote is not from the Ferrari WMS but a general description of how the downstream O2 sensor is used to control fuel trim. It is not unique to Ferrari. Pretty much every manufacture does it. Some manufactures were doing it before OBD II was introduced. I provided it as a reference so other might understand how the down stream sensor fits into the fuel trim loop. Ferrari uses the term "aging" because the drift in the switch point is generally associated with aging. But there can be other factors, like heat and vibration, which can affect the characteristics of the sensors. However, the point is that the ECU knows nothing about how old the O2 sensors are. The down stream sensor is always in the loop checking both cat efficiency based on a comparison between up and down stream sensors, and also looking at what is coming out of the cat. For example, if the down stream sensor consistently sees excess O2 in the exhaust gas (lean), but other checks indicate the cat is function correctly, the ECU will correct the fuel trim by making an adjustment to the shift point so that the engine is set a little richer. Likewise, if the down stream sensor sees too little O2 (rich) the ECU will move to a slightly leaner switch point. Now consider what happens with test pipes and extenders, or extenders in general. All the extenders do is trap exhaust gas so the signal from the down stream sensor doesn't follow the up stream sensor. When the ECU sees this it thinks the cat is working ok and no CEL is set. But that says nothing about the O2 content of the gas trapped in the extender and based on the level of O2 there the ECU may push the engine to a leaner or richer operating point even if the upstream sensor is functioning perfectly. Obviously if you have test pipes the exhaust gas comp will not be equalized and the O2 content will not be what it would coming out of a working cat. So there will be some adjustment to the switch point and hence the short term fuel trim. How that affects performance is anther issue.
Actually, installing the extenders on the rear sensors does not affect engine management. Management is the job of the front sensor. The rear is simply to verify that the cat is doing something by smoothing out the variations the ECU requires to handle both oxides of nitrogen and unburned hydrocarbons. I'm having the same codes, and will try the extenders on the rear. Car runs great; I just hate that damn light.
