There are simply too many possibilities here - best route is enlisting *hands-on* assistance from qualified personnel to perform the required testing / diagnosis.
Posting a list of the possibilities is not beneficial at all as the same *hands-on* testing must be performed for each.
Good luck!
RepairPal Certified shops can diagnose and repair your 1998 Honda Odyssey. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.
No quick / easy DIY repair here - - except perhaps checking engine coolant level. https://repairpal.com/repair-shops