if you don't install the secondary O2 sensor and you retain the OBDII ecu, then you will get the check engine light and one of several different codes.
Don't plug it in...get a code of P0141-heater failure.
plug it in but don't install it into the exhaust stream...you will get a code of P0420..converter failure <won't set right away-it will take a while>
neither of these will have any noticable effect of performance.
__________________
.
.
Liar, you couldn't cut me deeper with a knife if you tried
.
.
.
.