My 2004 Honda odyssey blows cold air in front vents and hot air in rear vents. This is with the heat on at highest temp for both areas. What could cause this problem.

SOURCE: top rear vents blow out cold air[ 2005 cadillac srx ]

Hello according the the manual, the back vent are to blew cold air only..the fronts vents are suppose to the entire car...

SOURCE: No hot air at rear vents on 98 Toyota Sienna CE

Possible that heatcore is going or hose from heater is leaking or falling off.
You can go to www.autozone.com and see a full free online repair manual for your van

SOURCE: 2004 Chevrolet 2500HD blows hot air out one side

check the fuse check the relay check the compressor and freon level

SOURCE: 2001 Windstar Hot air in rear cold air in front

there is a white box under dash that controls the hot and cold air. it has wires running to it. this has to be replaced.

SOURCE: front heat works, but rear blows cold air only

With your front heat working the most likely cause for the rear heat not to work is that there is an obstruction in the line that is preventing the warm coolant to get to the rear heater core or the rear heater core itself is plugged and not circulating. Another possible cause could be low coolant level where the system has enough coolant to provide the front system with enought warm coolant but not the rear system. Hope this helps