Left or right O2 sensors. I have green and red leds to tell me which O2 sensor I'm reading and a switch to switch back and forth from left and right O2. I added alittle humor with the Panic and Eject buttons. Panic turns on the ilumination of the gauges and Eject switches between the O2 sensors. Here's my setup:
CPARK Charles Park POWERTRIX Performance and Styling San Diego CA 619-944-6055