If I remember the diagnostic properly, I would check the level of the transmission fluid and possibly do a fluid change. At the very least, do a drain and fill. I did a drain and fill on my 2006 Camry and just changed the oil yesterday on the 2004 Quest. It’s pretty easy if you have ramps. Transmission pan is on the driver’s side; fill through the dip stick hole.
As far as the 400’s, I would have a local mechanic take a look at it. It could be the catalytic converter or something much simpler.