a quick test can be done with a digital thermometor like those used in meat packing. In a split second it can be determined if the cat is working if the outlet side is hotter than the inlet side. I have seen it done.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T