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.