[s-cars] O2/Lambda sensor question

[Paul Heneghan]

My mpg has dropped enormously over the last couple of months.  It's
dropped from an indicated mid 20s to about 20 (I presume these are good
old-fashioned UK gallons, not the puny little 80% US type).

First off, the car seems to be taking an age to get to temperature - in
fact channel 51 only seems to ever get to 79, even when the dash gauge
shows the normal temperature.  I plan on changing the engine thermostat
- I think I'm still on the original which is 10 years old now.  

I thought I would change the O2 sensor as I'm pretty sure the original
is still in there.  When I took the old one out, it was covered in soot.
Do I need to look at something else, or could the rich condition be due
to the cold engine, and a failing O2 sensor?

Can you scrape the soot off and reuse them?

What about MAF?  DO they need to be replaced as a matter of routine?

Paul
1995 S6 Avant
1996 S6