Central locking Shogun

Hi All

Sometimes when using the key fob to unlock the car the car immediately relocks

Try to undo passenger door with key and it relocks immediately

Unlock with key at drivers door all doors stay unlocked

Trouble is it only seems to do this when I don't have time to look at the car, or after dark, or when it is raining

I don't know where to start with this as I am not sure whether the car should unlock from the passenger door anyway If it should then I guess that would eliminate the GT alarm immobilizer and point to the central locking or maybe not

If it was the central locking relay why does the drivers door lock work with the key?

I have thought about getting an auto electrician in but the problem may not come back for several days and not appear when he is here.

Any help appreciated

Tony