False alarm

I have owned my 200SX Touring Auto for 12 years and it has done 48,000 miles. It is used most days for a few miles with weekly longer trips.

It is fitted with the Nissan Antitheft System II and has regularly been subject to false alarms when parked and resets itself after 30 seconds of siren with no further problem. The only thing which seems to trigger the alarm is bright sunshine resulting in a sudden temperature rise in the car, such as when the sun comes round. Nothing else such as rain, wind, frost, etc, seems to affect it, although it sometimes goes into attentionseeking mode on a dull day in winter without any obvious cause. The vehicle LED does not illuminate to report the cause when the ignition is switched on.

Neither the Nissan dealer who sold it to me, nor the respectable local garage who now service it, have been able to solve the problem. The system allows me to turn off the radar sensor – aka the movement detector – to leave a passenger/animal in the locked vehicle without alarms being set off. However, on the basis that the only other sensors are on the doors, bonnet, boot and vehicle inclination, it seems that it can only be the radar that is causing it. I am thinking of just turning off the radar to see if that works.

Your comments and advice would be much appreciated. John Scott Given your symptoms, I agree that the easiest test would be to turn off the sensor, so I would try this first. If that doesn’t work, it is still possible you may have a fault with a door or bonnet switch, or a fault within the loom. As bright sunshine triggers the alarm, does this coincide with the warmth of the sun warming up the bonnet, which may then expand marginally and alert the sensor? The alarm system may also be affected by a drop in battery voltage, so I would ensure that battery condition is good and that it is holding charge at a steady voltage.