Originally Posted by SudzRA

Here's a shot of a HFC for an Audi, with both pre-cat and post-cat bungs. The post cat OEM sensor would go to the post-cat bung (obviously ) and the wideband sensor goes into the pre-cat bung. There's the OEM bung for your OEM pre-cat sensor on your downpipe, and most HFCs for the RA/Evo X already have a post-cat bung, for the OEM post-cat sensor.



Here's a diagram of where each bung is in the system. Lolz mspaint



The sensor and gauge generally come together as a package. Usually between $200-$300.