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Anyone know the procedure for turning this off through the diagnostic connection. 1981 528I Owners manual states that this light will come on at 30K mile intervals indicating the need for a new cat. and must be reset. The blue binder speaks to a reset button on the service box but I'll be damned if I can find it.

Find the speedo cable junction box under the car/drivers seat area. Reset button is on the side. It's resessed into the box / easy to overlook.

Much thanks I'll look for it in the morning

And to imagine if you lived in Detroit this reset box just hung out under the car to get caked in road salt and a great big beehive of ice occasionally in the winter. Car design has improved greatly, the fun of the hobby still rests with the older cars.

Anyone ever seen an old Japanese car?

You'll need a small screwdriver or the like to push the buttons in, your finger will almost do it but not quite. And as long as you're under there, do yourself a favor, there are two buttons, push them each twice, one after the other. Failing to do that seems to bring the light on again much sooner than it should.

One more thing, it is to indicate O2 sensor replacement, not cat replacement.