You can also use an IR thermometer to find extreme temperature differential before and after the cat to determine if it's bad. Some bad cats get so hot they will even start glowing after a highway run.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280