I don't know if it has one or two, but if the new front sensor is an
aftermarket unit and not OEM, it could pose problems.

All OBDII cars have at least 2 O2 sensors. Take it to a service shop or
dealer who can test them as throwing parts at it will get very expensive....

There are front and rear sensors. You can go to Majestic's site
http://www.hondaautomotiveparts.com/ , put in the car's info, pull up the
exhaust manifold drawing, and see their location. The front one goes for
about $155 + s/h. The rear goes for about $194 + s/h.

Here's a nice summary description of what each does:
http://www.samarins.com/glossary/oxygen_sensor.html

IIRC, if the rear one fails, you risk destroying the catalytic converter,
which is expensive.

If the front one has failed, you should be seeing other symptoms, like poor
mileage, irregular RPM, hesitation, etc.

2. one staring you right in the face on the exhaust manifold, and one
after the cat, which is bolted to the exhaust manifold.

they need a new scanner then. IIRC, my code was PO135 primary oxygen
sensor heater circuit malfunction.

primary means top, the sensor in the manifold.

OEM honda, or aftermarket? either way its worth testing the sensor for
continuity. for me, the 2 bottom black wires read 0 ohms, which meant
the heater circuit was dead. or something like that. id bust out the
ohmmeter and helm manual and test em both.

yep, there are. but front=top and rear=bottom