Thanks, TN Tracker, for the pics of the hoses. I've had a similar problem with my Tracker, also a '96. Perpetual service engine light that usually goes out when it's warm outside. Once it gets cold or moist and cold, the light comes on. At first it coded as an "EGR circuit flow malfunction" now it also has an O2 sensor (pre-cat) malfunction.
Vehicles has always run fine with the exception of wanting to stall (very low idle) when it's restarted and warm. It's always done this. I checked the plugs recently after the service engine hasn't gone off for about a year and they're clean and "normal" looking ... not sooted up or anything. I also cleaned the EGR valve but it wasn't carboned up either.
Vehicles still runs well, though: I'm stumped but here in PA, the vehicle can pass emissions with less than 5K miles a year. This year, the light has to be out at inspection time because it'll be more than 5K miles. I've checked vacuum hoses, but will recheck. Thanks again !!