my '99 CA emissions 4runner has 142K on it. back in late 2003 when it had 61K on it, i replaced the bank one (front) A/F sensor in order to resolve trouble code P0135. about a month ago i got trouble code P0136. so i replaced the bank 2 (rear) O2 sensor. all fine for a couple weeks, and then i get P0420 (catalyst efficiency below threshold). could this be because the front sensor is bad again, or am i going to have to replace the cats? and, if the later, will i need to replace the front sensor anyway? i've read prior posts on this topic, and it seems possible that just replacing the front sensor might resolve the "imbalance" between the two sensors. obviously, i'm hoping to avoid replacing the cats. thanks in advance for any guidance!