As part of a routine service, the door lock mechanisms should be checked an lubricated. We have often seen problems with seized locks and broken handle cables caused a result of poor maintenance. Start with the outside:

Pull the handle out fully and look underneath. You will see the lock release cable. This runs in a tight-fitting sleeve and without grease, the handle can sometimes be hard to operate or even stick in the open position.

Spray some thin chain grease onto this wire and operate the handle 10 times to allow it to work along the cable.

At the same time, open the door and spray a little bit of grease onto the moving parts of the lock. Caution: do not overspray as you can cause the microswitch to become saturated and not operate correctly.

Should you have serious problems with the locks, have a look here at some more intrusive work.