Does anybody have ideas? I've just bought a Mondeo 2.0 TDCi Ghia 2002 everthing works perfectly apart from the passenger door remote locking. If the key fob is pressed to lock and double lock the car the car locks all of the doors bar the passenger front door. The car arms the alarm and has no issues. Only problem being the door is not locked.

The lock works perfectly fine mechanically if you lock it from the inside....so does that rule out the lock mechanically?

Wheres the first place to look?

sounds like you could have a problem with the passenger side solenoid or the power to it,until the door panel is removed and the power checked its difficult to say.