One problem related to check engine light on has been reported for the 1998 Subaru Legacy. The most recently reported issues are listed below.
1 x overheated on a hot day, heading up a hill. Noticed high temp and turned on heater, waited for car to cool, added h2o. 2 months later, driving on a hot day with ac on, sudden clicking, check engine light, lost power, smoke, stopped. Towed to garage that specializes in Subaru and they found hole in head gasket. Repairs to cost $3k and this is something they see 2x a week with Subaru vehicles. Found a web site where dozens of people complain of the same issue over the years. Subaru apparently has a recall on 1999-2002 vehicles, but not others with the same issue. Subaru should be paying for the parts on this widespread and known issue!.