Catalyst efficiency below required level

96 Caravan records error code 72 once or twice a month. No noticeable change in performance. The battery connections are good. Anything else I should check?

Check the cat itself. It may be going bad and causing an occasional light. It could also be the O2 sensor located behind the cat which reads the O2 in the exhaust stream after the cat has done its job.

A mechanic told me that if you are going to change a Oxygen sensor, that you should change them all. If you mix old and new ones (pre, post cat), you may get a false code the there is something wrong with the cat.

Thanks. How do I check the catalytic converter? My Haynes manual is vague saying to measure the intake vacuum to make sure the cat is not plugged. With only 70,000 miles I need to question why the converter would fail.