Pursuant to a recent thread on the number of oxygen sensors in the 1986 or 1988 Honda Pelude, I asked my mechanic about this. It seems the reason the '88 Prelude had two O2 sensors right beside each other had to to do with control over emissions. It's well known that OBD-II systems have two sensors, one before the cat and one after, but pre-OBD-II engines like the '88 Prelude's have two sensors for entirely different reasons. OBD-II cars use the second sensor primarily as a cat efficiency tell-tale. When a second sensor was used in cars like the pre-OBD-II '88 Prelude, it meant the engine management system had divided the exhaust into two streams, each with just two cylinders, and was monitoring each stream separately. The two INNER cylinders were monitored by one sensor, and the two OUTER cylinders were surveilled by the other sensor. Two sensors meant that Honda was able to monitor and adjust mixture and emissions twice as efficiently as it would have been able to with just one sensor covering the exhaust streams from all four cylinders at once. My guy could not remember offhand whether Honda did this just for the carbed versions of the Prelude or both carb and FI models. Furthermore, he tells me that the very latest cars are beginning to do this two-cylinder splitting again, but this time each pair has its own catalytic converter and pair of oxygen sensors. For example, some Ford V8s have FOUR catalytic converters and EIGHT oxygen sensors. Can you imagine owning one of these once the warranty runs out? I'll stick to my '91, thanks.
