Wideband O2 sensor ("shoulda bought this years ago")

One sensor most of the time. Some kits have two. The sensors need to be heated or they will plug up and die. Can't just leave them in the pipe with no connection to the box.

They work really well. If you want to see each side, place a bung in each pipe and swap it.

Dual plane intakes muck up adjusting of the carb to change how each side may read.