Apart forom the light coming on the dashboard telling you something you already know
Found this, personally i would just reset it :-
( From Michael A. Griese )
The Lambda counter is located under the dashboard
next to the steering column. The speedometer cable
passes through the counter (that's why there are
upper and lower speedometer cables). The counter
measures percent of use of the oxygen sensor, and the
sensor has a predicted 30,000 mile life. That means
that a Lambda counter showing 020 has travelled about
6000 miles since the last reset (20% of 30,000). The
counter should never vary more than about 250 miles
from the odometer. If it does, your odometer has been
tampered with or your car has had a rare upper
speedometer cable failure.
To reset the counter, remove it from the car and look on
the right side. You should see a small plastic disk with
two holes in it. Using a pair of snap ring pliers (needlenose
work also) rotate the disk until the counter reads 000.
You should also replace your oxygen sensor at this time.
http://mydelorean.net/html/howto/howto2 ... -Emissions