Thanks for that explaination, grcauto. Makes sense. At 85 years old and yars out of touch, I am not always (hardly ever) up to date. But I like to help when I can and learn on this site.When the cat first starts to degrade they will fail and turn on the light and after clearing the code it may take several times for the monitor to fail again and turn it back on. The times between good and bad become shorter and shorter until it'll code everytime. It's always a good idea to check the wiring to the O2 but scopes or graphing with a scan tool can give you good idea if wire chasing is more like chasing your tail.