The check engine light helps to give you a warning that some issue is pending with the drive train of your car, Thats all…unless it is flashing there is no immediate danger .
If it is flashing, it means that there is a good possiblity engine damage will occur if you do not shut the engine off right now.
So here is what I would do.
A lack of oil is the most damaging to an engine.
Check to be sure the oil is full. If it is…cancel the appointment with the dealer.
Keep a close eye on the gauges untill you get this resolved.
Next go to any Autozone, or Advanved Auto parts store and ask if they could read the codes.
They will give you a printout of the codes and they can advise you if it is an issue that must be attended to ASAP or if you just need a mechanic to look at the car soon. All for free.
Then post back here with the codes and any symptoms so we can try to advise you more.
It does sound like the oil change mechanic diagnosed that it is an emissions problem. These can be as simple as a gas cap left off, and rarely are a major issue that needs attention NOW!!!
One thing that people do that damages the emisions system is topping off the tank after the automatic shut off stops pumping gas. So if you top off your tank… stop.
I would advise you to find a independent mechanic…ask around for reccomendations.
Dealers have a much higher cost than most independents and charge more in turn.
Your local OIl change garage may have a good mechanic on their staff, but the chances are good the guy that will work on your car will be someone that was taught to change oil, look for items to upsell while your car is there, and knows little about diagnostic’s. They are well known to forget to tighten drain plugs, and often throw part after part on the bill…trying to eliminate the problem instead of properly diagnosing it. Do you really want that guy working on your car!!!