Gas and Oil Home Heating Furnaces - Zone Control A/C

I have a Carrier w/zone control by Parker Electronics.Heat Pump.It has two controls,upstairs and down stairs,I noticed that the up stairs vents have next to know air coming out of them.The vent is cold.The down stairs vents are blasting air.I have been experencing a pressure sensor out of range code intermitantly for the last 1 1/2 years.I thought I fixed that when I discovered the blower locking up.Ido think the code came once after that,and went away again.I went looking for the zone doors,I found two doors.One of then had a big box attached to it and some electronics that had something to do with the set back feature.Disconcting and maully turnig either one of these doors seemed to do little to airflow upstairs(if any) but seemed to be able to slow down down stairs w turning either door.May be it takes a certain combo to work the upstairs?Can someone explain the operations of these doors to me? can I override them? Is there a diagnostic procedure that can be outlined without alot trouble?Any input would be welcomed.

Sounds like there is another door (or doors) that are in the branch of your ductwork that takes air to the upstairs zone. The door must be closed for some reason. (A tiny amount of cold air leaking by the door is keeping the vent cold.)

Step one is to find the closed door. Follow the ducts. The door will be upstream of the problem zone.