Alarm keeps going off every few minutes?

I have the 9495A and its been faultless for a long time now for some reason I cant reset it. If I just enter the code and don’t press reset it stays off for about 5 minutes before it goes off again. I cannot press reset without it going off again although it will stop temporarily if I enter the code. I have taking all the batteries out of the sensors to eliminate them as a cause of the fault. I have also replaced the internal battery but still it keeps going off unless I turn it off at the wall and disconnect the battery.
I dont have the external alarm fitted now as it got broken. Would this cause it or should it still work without a external alarm? please can someone help.