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My Cobalt exhaust came with a bung hole after the cat. Intrigued as to what that could be used for. I would have thought all sensors (wideband etc) would require fitment before the cat.

I seem to remember that California has/had different emissions laws than the rest of the US, and they required two sensors, one pre-cat, one post.

I think the reasoning was that if the cat has failed, the two sensors will match readings, and flash a warning light on the dash.

Systems on most cars these days have an O2 sensor before and after the cat.

hks_kansei wrote:I seem to remember that California has/had different emissions laws than the rest of the US, and they required two sensors, one pre-cat, one post.

I think the reasoning was that if the cat has failed, the two sensors will match readings, and flash a warning light on the dash.

Correct

The NC has something like this

different markets also introduced tighter emissions controls in different periods. USA for example just on OBDII and tighter controlls in '95. took us up to 3 years later to have the same gear in various mazdas. so you'll find some usa made stuff in the mid 90s have extra bits and bobs.

That all sounds reasonable.