What do the plugs look like? The o2 will almost always have soot on it to an extent. However the fact that your o2 is covered on one side heavily does indicate a rich run situation.
Having a lean reading at the computer with an actual rich condition at the engine screams exhaust leak before or near the oxygen sensor.
You should have a minimum of 18” of pipe after the sensor before the pipe exits to atmosphere. Another thing that tends to trip people up is using the clamp on o2 bung supplied with the unit. It’s radius is made for a 2.5” pipe. If you are clamping it to a 3” or 3.5” header collector, it will almost never seal.
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-Jason
1969 Pontiac Firebird
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