If you think your cat is borderline, then instead of a 20 minute run, make it an hour or more. That'll get the cat good and hot, which might push a borderline cat into an OK reading.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.