Well, I finally got back to the Subaru. The fuel filter was original, so I replaced it. Then I looked to implement your suggestions. Before I dug in, I looked for anything obvious and noticed the spark plug wire on cylinder 3 was not all of the way on the plug. I had my young son reach his small hand in (engine cold) and push the wire back on. (Not much room in the engine compartment.) We took it for a drive and it ran well, but the Check Engine light was still on. It was late so I parked it for the night and checked the next day. Again it ran well, but this time the Check Engine light went off after a few minutes! I've been driving it ever since, it runs well, and the Check Engine light hasn't come on again. It appears either the new fuel filter or the plug wire was the culprit...I hope.
Thank you all for the information and advice.
