When I lock my 4 yr old 406 HDI 90, the locks sound as if they are locking
but immediately they unlock again. The dashboard display shows that no doors
are open.

I think the problem is in the front passenger door or the boot.

If I play with the locks on these two, then the car will start locking OK
again.

Of course, since the key loses the sync with the car after a few attempts, I
cannot keep trying this until the car does decide to lock!

Can anyone shed any light on the problem.

Thanks.