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Cat Bypass and Poor Mileage


Model: 964, Year:1991, Mileage:82000, Type of use:Street use only
I have a 1991 C2 Coupe and recently installed a cat bypass. The sound is terrific but the mileage has decreased by half and there is a strong exhaust smell. I did not install a new O2 sensor, but gently brushed off the original sensor (I assume). Also, disconnected the battery before the bypass install so the DME could perhaps adjust if needed for the bypass (though O2 sensor is before the cat would start its job, right?). Do you think a new O2 sensor would cure the matter or?The car idles like a dream, so at least that is not an issue.

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