heating control panel showed error

Hi I am new to the forum and have some issues with my 96 GCJ.
Has anyone ever get an error on there heater control display? I have been having problems with my heat which I thought was my heater core which I have had serviced but my heat comes and goes. I have had an error come up on the heater control panel and it showed an error a couple of times. it would be set to 21 degres c and then it would go a bit cold I then set it to 22 dc and it would go warm for awhile then it would go cold. I set it to 23 dc then it would go warm for awhile and go cold again. Personally it is driving me crazy, while doing al of this it is on manual heat not automatic other wise I liked it on automatic but it started with this heat problem. How would I be able to check the heater control unit to see if it is the problem or if the control sensore ( if that is what it is called) under the dash by the fan is wacked how would you check it? any help would greatly be appreciated. thanks



Rob