^ x2 I've heard low on refrigerant can be the cause, with reasoning something about the specific way the evaporator coil is mounted, a temperature gradient across it (due to low refrig) ends up with passenger side warm, and other vehicles evap is mounted differently so that doesn't happen. I've also heard explanation it could be one of the inside air doors broken. Mine does same thing but seems intermittent, seems to change (pass. gets colder) sometimes when I go over a bump. My truck isn't driven in winter; plan this winter is to tear the dash apart and do the door upgrades. Not looking forward to that. The Cummins is great, but comes with a Dodge wrapped around it... .