Cant reset alarm

I have a Simon XT.
While I was setting it up and testing it, I triggered an alarm in the kitchen, which is a PIR sensor in SN 4 position. Now there is permanently an alarm status associated with SN 4 when I hit the STATUS button.
I tried hitting STATUS and then the DISARM buttons, as it says in the manual for clearing an alarm status, but it won't clear. I also tried hitting STATUS and then entering the master code, but that just performs a subdisarm.
If I move the sensor to a new SN position, I can get a STATUS OK when I press the STATUS button, but as soon as I move the sensor back to SN 4, the alarm status comes back.
Basically, it seems to me that SN 4 is now useless. How do I reset it? I don't want to have to abandon a sensor position every time it's had an alarm.

thanks

john