If you have an Ohmmeter, first test the electrics by checking the injector plug to see that it is getting electricity while the engine is running.
The 850 injectors are so easy, they just pull out with the rail. The 1998 changed, the top of the injector rail has a metal plate that holds on to the injectors, so all of the injectors must be pulled out at the base before you can remove them from the rail. Your S40 may be different, and probably is.

There is a "special tool" to remove the injector. I use a 17mm wrench to pry it out from the manifold, pulling with my left hand at the same time. You may or may not need to replace the O ring at the top and bottom, depending on how it comes out. When putting it back in, just push the injector down until it feels snug. It's not rocket science :)

Good luck,

Klaus
--
1995 854T and 1998 V70R