The central locking door lock mechanisms on Saab 9-3s (and 9-5s) are known to occasionally malfunction/stop working. When you hit the central locking button on the remote, the affected door often won't lock (or won't unlock if it was already locked). The issue tends to start out intermittent and then worsen with time.
A new unit is currently around £140 (without labour), but they can be obtained second hand on ebay for around £30. Replacing them is incredibly easy.
It is also possible to strip the unit down to repair it, however, it's not without it's risks. While the door lock unit is removed the door won't latch closed (and certainly won't lock), so if you damage the unit trying to repair it, you're going to be in a tricky position. The route I took was to buy a second hand unit from ebay, fit that and then look at repairing the original (so I've got it on the shelf if the replacement starts exhibiting the same symptoms).
This documentation details how to dismantle and repair the locking unit mechanism