What could cause this and how would I fix this problem ?
I have dual heating heat only on the drivers side and cold air on passenger side on 2008 Ford F-350 Super Duty
We can offer a guess but this really needs to be tested by a qualified mechanic with the proper scanner. It is possible the heater blend door or it's actuator is having a problem but that is a guess. Scan test should show where the trouble is or at least the right direction to take.