Scantronic 9800 shows a zone as open but it isn't?

Hello and asking if someone can help.

Yesterday morning was able to set the alarm as usual. Later in the day it would not set as showing a zone still open. The number indicates it is the up and over garage door. Apart from the door was opened once nothing else has happened during that time. The sensor is in its usual place. I have tried waggling the door etc. Have taken the sensor off and placed it on the contact and also closer to it. But still the zone open display.

The system is early 1990s but was seen by an engineer two years ago as I wanted to move something and he fully tested the system. I have put a call out but will be a week before an engineer can attend on this occasion.

I have tried to omit the zone without success. Enter access code, then setting letter, then press omit, then the zone number either on its own or with a zero in front. Pressing omit seems to make no difference. I do not know if when installed if programmed to 'non omit' that zone.

But curiosity is getting the better of me. Why malfunction without some warning and just like that? Is there anything I could do to try spot what the problem is?

I have the original user manual but have failed to find an installation or engineer manual. The installer is no longer around....

Thanking you in anticipation.

Regards