Replace the Bank 1 Upper sensor. DO NOT replace the Bank 1 lower sensor as you will be wasting money and time.

Edit. Per TSB,
"When the downstream switch rate crosses a threshold value (approximately 0.75 switch ratio), a code is stored (P0420 and/or P0430) and the MIL illuminates (refer to Figure 5 and Figure 6 ).
NOTE: IF A CATALYST IS DETERMINED TO HAVE LOW EFFICIENCY AND REQUIRES REPLACEMENT, REPLACEMENT OF THE DOWNSTREAM HO2S SENSORS WILL NOT BE NECESSARY."

http://www.v8sho.com/SHO/TSB0197HO2SServiceTips.htm


Last edited by Tony2005; 06/20/06 12:27 AM.
"Always do the cheap and easy ones first." 1996 V6 ATX 96K miles