Hi,

O2 sensors are often available as an OEM replacement (includes factory
connector) or a universal style (where you splice the wire.) Avoid the
universal style whenever possible--the length of the wire and integrity
of the connection are problematic in some apps. Hope the one you got is
the OEM style.

Dunno about your car, but the older Subies used the screw in style
sensor. It requires a spark plug type socket to remove easily. The
socket has a slot in the side to allow the wire to be pushed out of the
way. Auto parts stores have 'em--they're not that expensive.

The sensors sometimes get almost frozen in place. Warming the exhaust
sytem (doesn't have to be real hot) often helps you break the old sensor
loose.

When installing a new screw-in sensor, some anti-seize compound should
be used on the threads. Many sensors ship w/ the compound already
smeared on the threads. Make sure you DO use some a/s compound or you'll
REALLY get the frozen sensor blues next time you try to remove one.

Many people put a bit of dielectric grease on plug-in electrical
connections when they install new bits. Subaru even suggested it at one
time as they were having corrosion problems w/ some underhood
connectors. A caution, though: I used to subscibe to a newsletter that
had "topic of the month" kinds of info. In one, the author warned
against using any grease or other substance on O2 sensor connections.
The reason was the insulation on a lot of the wires is extruded w/ tiny
air vents lengthwise thru the plastic (for temperature control), and
foreign substances in the connectors can clog them. The cooling effect
is compromised, w/ attendant errors in reading the voltages by the ECU,
which might result in running problems.

Good luck!

Rick

What code did you get. Did you try clearing the CEL and see what code ,
if any , came back? How many miles on the car. How do you know the rear
sensor is bad?

Carl