Follow up,

Whay I di was disconnect the battery for 30 min plus. I also tapped on the O2 sensor and surrounding exhaust pipe to possiblt rid any particles that may have stuck. If any.

The CEL has gone off and so far (10 miles) has stayed off.
However, if a fault exists, I can expect it to go on again after 30-60 miles.

I'll follow up.

Thanks for the DIY meter link and opinions !!

__________________
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