I'd recommend having a smog check repair center inspect the fuel injection
"feedback" system. This will ensure whether the oxygen sensor is working
properly, your Honda's emissions computer is receiving the signals from the
oxygen sensor, the computer is computing the data properly and sending the
correct signals to the fuel injectors to either increase or decrease fuel
delivery to the combustion chambers.
The feed back test will utilize a 5-gas analyzer (usually a smog machine)
and propane. The smog technician should introduce propane to the intake
system and ensure the oxygen sensor is reading this introduction as an
increase in CO, and ordering the computer to decrease fuel delivery at the
instant propane is added to the system. This test should be done after the
engine has sufficiently warmed up and while it is running at idle. If no
change in fuel delivery is seen we know we have a feedback problem. Then the
diagnose should turn to which part of the feedback system is defective.
Using a voltmeter the technician should observe the voltage output of the
oxygen sensor as propane is added. Voltage should increase. If there is no
increase we have a defective O2 sensor. The sensor should be replaced and
the test started over. If voltage increase is present at the sensor output
the technician needs to ensure voltage increase is also present at the
computer input terminal. If voltage is present at the terminal as well next
the technician must ensure the computer output terminal to the fuel
injectors are lowering "injector pulse rates". At the end of the feedback
test it will be know whether the high CO fault is electronic or mechanical,
and the technician can further diagnose the fault.
You can either go this route, or change the oxygen sensor before a diagnosis
since it's long overdue already, and get a third smog test with new results;
maybe it passes. Although you've mentioned all other emissions are within
limits, even a slight increase or decrease in CO (caused by a defective
oxygen sensor) can alter HC emissions.
As far as high octane gas... Using it might help lower NOx but only if your
vehicle suffers from this failure. High octane burns cooler and slower
(keeping NOx low), but it also produces less power which your engine might
during acceleration. This could cause other emissions (HC and/or CO) to
increase. It is recommended to use only the manufactures required octane at
all times, specially during a smog test. Your Honda Prelude's engine
compression is designed for optimal fuel combustion with the 87 octane fuel
and its emissions computer is programmed to process information based on 87
octane gas.
