Sixto, to replace valve seals, valve cover has to come off. Also valve springs and keepers as well. You will need valve spring compressor and air chuck adapter to blow compressed air into the cylinders to keep the valves shut whilst removing springs and replacing seals.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.