someone wrote:
> My stock `00 Civic needs a O2 sensor, is a special tool needed (question

not necessarily. when my upper one in the manifold took a dump, i knew
it was bad. if it hadnt come off normally, i woulda snipped the wire and
put a 1/2" drive socket on it. to reinstall, a regular open end wrench
was all that was needed.

if the area is too tight to wrench, then id go with an O2 sensor socket
to install.

> two, should I replace both O2 sensors (matched set?) even though only
> one of um is bad) ?

hell no. i did change the bad one same day, because i didnt want an
overly rich default mixture ruining the catalyst and/or the secondary O2
sensor.