Perhaps the vents motors need recalibration. There is a procedure to do this, but I can't remember how. Something about pressing the recirculation button a certain number of times. If out of calibration, the motors will not open (or close) fully and could cause the problem you are describing. The dash does not need to be pulled, it's an automatic process.

By the way, there is no heater valve. These valves are a weakness, often breaks and leaks out all the cooling fluid. So the S40 has a radiator that always has hot water circulating through it and a flap to select between hot and cold air.