sluggish when shifting

The service engine light goes on when the On Board computer diagnostics detect an (emissions system control) error condition. It then stores the error code which can be read by an OBD II compliant code reader. You really will need to get the error code to know why the SES light went on and hopefully that is the source of your problem. If you live near an AutoZone, they perform the code reader service for free and will tell you the error code which can then be looked up to determine what the problem is on your particular vehicle. Since you do not indicate the mileage or maintenance history and only can say it acts "sluggish", then there is insufficient information to even venture a guess. That said, 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 and post what you find for others benefit.

Bob

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