[sraney]

Duffy--since you're kind enough to read our comments and provide your learned responses, here's a follow-on to this one. As I understand it, the forward sensors are used to help calibrate the operating mixture, and the rear sensors (unheated?) are there simply to monitor the health of the catalytic converter(s). However, from this gentleman's experience (and from your previous comments about probably ending up replacing all the sensors to finally stop the codes), it appears that there is some kind of comparison testing not only side to side but from front to rear, even though the rear ones aren't directly involved in the mixture calculation. Any thoughts? Thanks.