I'm not a mechanic either, but have had a few check engine lights come on over the years. I'd get second opinion before forking over $250 for a sensor replacement. I've had mechanics check the code, then clear the warning light and wait to see if the light comes back a second time. Often it doesn't, and is the result of a single incident that wasn't a huge deal. I also doubt the sensor costs more than $150, or there's even a full hour of shop time replacing it.
I've had my '08 Ram 2500 light come on twice in the last five years for a loose gas cap (or so the code said), but recently forgot to put the cap on, and left it swinging in the breeze for about 40 miles and it never tripped off a warning light. Go figure.
About six months ago the check light came on in our run-around hoopty, and before I got around to plugging in the OBDII to read the code, a week or so went by and the light went off on its own.
Come to think of it, the only sensor I've ever had replaced was in my Tacoma, at about 50K when the TPS started acting up. It really worried me because I got freeway speed hiccups and the computer couldn't capture the code the first time. When it finally did, it was a simple sensor replacement, and I was glad to buy a new one.
Again I'm not a mechanic, but I've learned to be distrustful enough to ask for second opinions. And to answer your question about Seafoam, it shouldn't make the problem worse, might make it better, or might correct itself with a fresh tank of fuel.
