It's not hard, I just did it on a 93 900s.

1. The sensor is located between the manifold and catalytic converter.
As a point of reference, from above find the manifold and look down to
where it connects with the muffler pipe. After that, get under the car
and look where they connect and again follow it about four to nine
inches while looking on the top side of the pipe. You should see
something that resembles a spark plug in diameter with some wires
going up toward the engine, that's it. More specifically it will be
near the passenger side front wheel, near the rack.

Note: It might be hard to break free from under the car as you'll not
have much leverage but with some effort and something to extend the
length of your wrench (gives you more leverage) you should be able to
do it.  You might want to use some type of spray to help loosen the
sensor once you break it free. Keep working it back and forth so it
can work its way in.

My sensor was about $80 (non-turbo) at Advance Auto Parts. They might
give you a few choices of sensor; I took the one that was
multipurpose.

2. You'll need to follow the wiring harness up to where it connects,
unplug it.

3. Back under the car; cut the wires as close to the old sensor as you
can. Don't try re-wiring under the car unless you have something to
get it high enough to really work.

4. Pull the harness up top but make sure you follow the path so you
can send it back down the same way.

5. The sensor will come with a package of anti seize, three wires (1
black and 2 while), connectors, and a shrink wrap sleeve for a water
tight seal. Simply connect the wires back together (up top), clamp
them down, pull the sleeve over and use a hair dryer on high heat to
shrink the sleeve.

6. Cover the sensor with something clean and fish it back down the way
the harness came up.

7. Get back under the car again, put some anti seize on the threads
(not on the sensor probe) and screw it back.

8. Go back up top and twist the harness counter clock wise to get the
tension off until there's no pressure on it from screwing the sensor
in.

9. Reconnect the harness and you're in business.

10. Disconnect the negative pole of the battery to clear the code
(check engine) and reconnect it.

It should take about 80 miles of driving for the computer to full
register all the new information but you should see a difference in
the way the engine runs sooner.

If the check engine light comes back on, check the Air Mass Meter.
It's located between the intake and air filter. It's very easy to get
too and you can do two inspections that are both also easy.

The first is to just visually inspect the inside. It looks a lot like
an incandescent light bulb with the small thin wires. If any of them
are not connected it's bad. The other test is to disconnect the Air
Mass Meter but not the wiring harness, let the engine warm up to
normal tempter, turn the car off and check to make sure the wires heat
up to a glow, if it doesn't it's bad. The sensor is very expensive. I
called the dealer and it was $425 but there was a place on the
internet I found that had a remanufactured one for $175.

Have fun and good luck!