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If you are talking about north american cars, the early cars have 2 cats and 4 O2 sensors,with the cats in the midpipes. The later cars put the primary cats in the headers and a second set of cats where the primaries used to be. These still have 4 sensors, up and downstream of the primary cat. There are no O2 sensors after the second set of cats in the midpipe of the later cars. That is why you can eliminate the cats in the midpipes of these cars without CEL issues as the levels are not measured after the secondary cats.
todd
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Current Cars:
1989 911 Targa
1984 944 Original Owner
1971 911T
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