Pursuant to a recent thread on the number of oxygen sensors in the 1986 or\n1988 Honda Pelude, I asked my mechanic about this.\n\nIt seems the reason the '88 Prelude had two O2 sensors right beside each\nother had to to do with control over emissions. It's well known that OBD-II\nsystems have two sensors, one before the cat and one after, but pre-OBD-II\nengines like the '88 Prelude's have two sensors for entirely different\nreasons.\n\nOBD-II cars use the second sensor primarily as a cat efficiency tell-tale.\nWhen a second sensor was used in cars like the pre-OBD-II '88 Prelude, it\nmeant the engine management system had divided the exhaust into two\nstreams, each with just two cylinders, and was monitoring each stream\nseparately. The two INNER cylinders were monitored by one sensor, and the\ntwo OUTER cylinders were surveilled by the other sensor.\n\nTwo sensors meant that Honda was able to monitor and adjust mixture and\nemissions twice as efficiently as it would have been able to with just one\nsensor covering the exhaust streams from all four cylinders at once. My guy\ncould not remember offhand whether Honda did this just for the carbed\nversions of the Prelude or both carb and FI models.\n\nFurthermore, he tells me that the very latest cars are beginning to do this\ntwo-cylinder splitting again, but this time each pair has its own catalytic\nconverter and pair of oxygen sensors. For example, some Ford V8s have FOUR\ncatalytic converters and EIGHT oxygen sensors. Can you imagine owning one\nof these once the warranty runs out?\n\nI'll stick to my '91, thanks.