The first (upstream) sensor monitors fuel trim for the computer

The second (downstream) sensor monitors the performance of the catalytic converter.

For a V-6 under OBD-II (96 - on), this scheme is present for each bank of the motor.

Steve


98 Contour SE Sport 2.5 Duratec ATX The wifey's car 89 Taurus SHO - 246K miles 94 SHO ATX - 190K 1997 F-150 5.4L ATX - The Workhorse 150K. ANY THREAD WITH "OMG" or "WTF" ETC IN THE TITLE WILL BE IGNORED!