AutoZone is on the way to most of my city errands, so when I have such a light I just have them check it for free. Same with simple stuff I have them put on new wiper blades for free (In reality I hate to fiddle with them, but I use the excuse my hands are always "sore" so that
I just seem like an old guy without dexterity any longer 
) I'll change some car batteries yet. That's all I do any longer. The rest is too much trouble.
The last time at AutoZone he told me the gas cap was probably just loose, so I sealed the gas cap o-ring with my swimming pool o-ring lubricant and it went out. I also have a relative into mechanics with one of those testers, that is, if you can get a hold of him. But he did use it on various occasions when one car kept getting those lights.
I suppose $50-some is worth it if you will fix things yourself, some guys and gals here do. But for someone like me, AutoZone is good since it's free.