IMHE the SES light and the sometimes resulting error code seems to have been
designed by Micro$oft in that when the computer cannot determine an obvious
or simple fault then it issues a more or less random error code.  My
favourite is the faulty cam position sensor code since the S series engines
do not have a cam position sensor.  I agree that getting the codes read at
AutoZone or other free source is a good place to start but IMHO they should
not be accepted as gospel truth.  If you are lucky, the error code will give
you a starting point for diagnostics but do not accept them as a final
diagnostic.

IMHE an SES light has most often been caused by a poor plug wire or aging or
poorly tightened gas cap.  I pick up spare gas caps when available and find
that simply exchanging the cap often results in finding one that the SES
light likes better.  I am too cheap to have tried a new one.  Sometimes I
suspect that overfilling the gas tank will lead to random SES lights which
seem to light with no observable performance symptoms and go away with
cooldown and restart.  I suspect that overfilling causes error codes from
the vapour recovery system.

Plug wires can easily be tested with an ohm meter and spare wire sets can
also be obtained inexpensively at the DIY wrecking yards.  If they look good
and test OK then they are probably not your problem but flex them a bit when
testing to look for intermittent poor continuity.

The coolant temperature sensor is often reported here as causing most of the
poor performance issues and it is a simple and inexpensive item to change.
Personally I am still running the original CTS but YMMV.

I have one S series car which seems???? to run MUCH better and with MORE
power AND smoothness with the SES light on than off, and that the light
cycles on and off coincident with change from city (off & poor) to highway
(on & good) conditions.

IMHO, Monitoring fuel consumption carefully, consistently &  ACCURATELY,
with good record keeping, can be a good diagnostic indicator of system
performance.

When a performance problem happens right after servicing it usually means
that something was disturbed, and in this case I would suspect the computer
does not like the plugs or the wires.  Check that the plugs are correct, you
may have got one that is faulty, good luck finding it.  If you had done the
service yourself you would have the tools and experience to just buy one new
plug and change one at a time till you eliminate the problem.  I doubt the
dealer will admit responsibility and will charge you big $$ to look for a
new problem and may try to sell you new coils or similar.

IMHO, Doing your own servicing will allow you to become more intimate with
the cars condition and will usually also mean a more careful job as well as
saving big bucks and lots of time.  I agree with Bob that "most times if the
vehicle
has been properly maintained, the fault is usually something fairly simple
and easy to replace, like a sensor, a vacuum hose, or the EGR valve."

Good luck, YMMV