What are the specifics on the car? year, mileage, etc. The down stream oxygen sensors monitor the catalytic converters, when things are working normally they generally deliver a pretty steady value around .700 mV +/-. This can be observed on the average scan tool. When the downstream oxygen sensor voltage starts to vary widely in a similar fashion to the upstream sensor, that's an indication the converter has about had it. I would make sure the codes point to both converters in case the recommendation is to replace them in pairs although only one has failed. They're not like shocks. Depending on the mileage, you may want to look at oxygen sensors also, sometimes they cause false converter codes.I had my check engine light come in and I was told both catalytic's needed replaced and they had to order parts. OUCH!!! I love my caddi sooo it is what it is. I was coming home from work and noticed the light is off, It is still off. You know it just makes a person go HMMM. Any explanations?