This car is a California version and does have a connector for a diagnostic tool. I have not counted the pins.

The o2 sensor was replaced about 2-3 yrs ago and not many miles have elapsed, say 8K. This does not exclude the O2 sensor of course.

The owner manual also implies that the O2 sensor may be at fault when the CEL goes on but says to have it checked, apparently for other possible stuff.

Can STP injector cleaner cause this? One std. bottle treatment in one full tank.

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD