Emmisons control system light

2001 Mercury Sable, 110K miles, had transmission replaced 3 years ago, had a complete tune up done 1 1/2 to 2yrs ago. Recently had A/C charged and filled.

My car began sputturing a few days ago while in park & while driving. Then shortly after the emissons control light came on. Blinked a few times & then stayed on. This has repeated for the past 3 days.

I put some fuel injector cleaner in hoping this was a result of poor fuel quality. I have driven for 3 days & it has not improved.

I'm assuming my next step is to see if it is a spark plug that is misfiring or something. It isn't constant & it ususally only does it while driving through town not on the highway. Also it seems to get worse when I'm parked and have the A/C on.

Just would like to know if there are any simple test I can do at home before taking it to a mechanic & finding out it was a simple repair.