STI Wideband

I know this is a super dumb question, but I have a 2015 sti and was wondering which o2 sensor should I replace with my AEM wideband sensor, and will that take out the option to have closed-loop fuel control for cruising.

Is this a standalone Lambda sensor and gauge that you are installing? if so, the best option is to install a new lambda bung into the downpipe post turbo and leave all of the existing sensor in place. This allows for the OEM ECU to use the sensors to control fuelling.

If you have an aftermarket ECU fitted that is going to be using that sensor for lambda control, then replace the OEM sensor in the downpipe with the AEM sensor and wire it into the ECU.