Stuck my fingers up the tailpipe of my car and probed!

Took my fingers out and they are black and sooty.

Also, the is a rim (1/2 inch thick) of block soot on the exterior of the tailpipe end (wierd).

Is this an indication of running rich?

I read somewhere that to test the O2 sensor, one could disconnect it and see if there is any change to the engine running.

If there is no change with the O2 sensor disconnected, the sensor is fried.

Is this test valid?

Also, is there a convenient way to disconnect the O2 sensor from the engine bay or cabin?

In any case, the switching from Sport mode to Economy is probably coincidental. I don't think that was the cause of the problem.If the car was stuck in S mode, it would start from 1st gear - but it doesn't; it starts from 2nd gear but now there is more power.So I believe the car is running rich.

Also, at idle, I can hear a mild fluctuation of the engine but I think it is very slight that I don't see the fluctuation on the tach.

Help!