Paul, a settlement crack will nearly always run in a straight line from the top to the bottom of the wall. That sounds like what you've got.
An expansion (also called contraction) crack usually runs in a stair-step patter diagonally. So, we know it's not an expansion crack.
Other than what Ron suggested, here's my experience on it: If your house is new (which I gather it is not) I would give it a year to see if the settlement causes it to crack any wider than it it is. Also, in that length of time the the settlement should be over with, unless: 1) your ground has dried out to the point that the foundation is starved for moisture, and that's part of the problem. During droughts it's a god idea to wet your foundation-line weekly. Or 2) the ground beneath your foundation has a void from water seepage beneath the house. This will undermine the foundation and cause it to weaken. You can check that by digging down at the crack-line with a posthole digger and seeing if water is present beneath the foundation line.
If you decide to fix it, here is a remedy: Dig out a square (extending 6 inches outside the foundation line, 6 inches or more back toward the house-line; and 6 inches to the left and the right of the dug hole) and a foot deep under the cracked area (on-line with the crack). At each corner of the square hole you've dug out, insert cut lengths of 1/2 inch rebar extended 6 inches below your depth-line of the hole in each corner and mid-way 6 inches of each other; then one in the middle. Next, cross from each rebar horizontally, and tie them with tie-wire. After that's completed, mix up some bags of redi-mix concrete in a wheelbarrow and pour the void all the way up to the foundation. Pack it with stick so that it settles in good as you do the slush up to the foundation line.
That technique will stop the settlement crack at that point of your house. The next step is to fill the crack in the existing wall.
I always chisel out a V shape along the crack about 1 inch deep and the width of the chisel (small side). Then I use a paint brush to dampen the the crack. Next, I mix up some portland cement - only, no sand (or you can use an additive like "Bondex" or "Welde-Crete to make the bond hold) and slurry it in the crack. Then, I mix 3 parts sand to 1 shovel portland cement to 1 part lime (lime helps it to stick better) and repoint the crack half-way out. Allow it to stiffen but not come to a complete set, then slurry again and finish filling the crack. As it begins to set again, I use the paint brush dipped in water to feather the cement until it mathches or blends with the wall. You can also do this with brick. It saves a costly job of cutting out old masonry and laying in the new brick. And with brick, you can go to a paint store and match the color (or colors) and touch up the area of the cracks. This is especially useful where masons have used exceptionally strong mortar, whereas it is impossible to cut the brick out without considerable time consumming work to save the undamaged brick.
If I can be of further assistance, please feel free to e-mail me. Good luck with your foundation problem. I hope this helps. --- Jack