How do you repair diagonal wall cracks?

I would have the foundation checked.

When you have diagonal wall cracks that appear/disappear with the seasons, it is a foundation/soil problem. Have a professional check it out.

I agree with the other people that’s advise you to check your foundatoon for problems that’s probably caused your cracking in the first place. But to fix your crack your best bet will be using a paintable silicone to fill your crack and use a putting knife to smooth out the silicone with your wall. If the crack if very wide you may need a wide putty knife to smooth out the silicone with the wall. If you crack is very deep you may want to fill it in slowly using few passes letting each pass dry before adding the next pass of silicone. If you put in a thick area of silicone it won’t dry well in the middle of your silicone and will crack too. So you may need to run a thin layer of silicone in your crack if it’s a deep wide crack and let each layer dry before adding another layer till you get enough silicone to have the silicone flush with the existing wall that you’ll be able to use your putty knife on the last layer to make smooth with your wall. You can use a damp rag over your putty knife to help smooth out your silicone too. Once it’s dry, it you have a textured wall you can use a can of texture spray to feature over the silicone to match your textured wall and again let the texture dry. Once dry you can paint the whole wall and your crack should be covered and hidden well. The silicone has some elastic value that will stretch some with the give of your wall. But remember this is really only a bandaid to your real problem. Your having some kind of settling issues going on for it to be cracking in this manner in the first place? Either your foundaction has failed or you may have a sink hole starting that’s causing your foundation to fail? Your going to need a professional foundation inspector to check and see what’s going on that’s making your home settle so much? This could get where your home may not be safe to live in if your home keeps dropping? Water line, gas lines, can break with too much setting too. So this can be a very serious issue if the settling continues? I’ve seen homes haven to be torn down completely from sink holes. Hopefully your problem won’t be this extrem, but should have it checked out ASAP to make sure it’s not a issue that could be dangerous?

Rack it out - Fill with expanding foam. When dry, sand it off. monitor it and if it happens again, check if your foundations are moving with the seasons too!