Not every microwave has a wave guide cover as such. However if your microwave is sparking inside the most common cause is a bad Wave Guide cover. However dried up and burnt food particles as well as if you see rusting on the inner liner this also can be the issue . If you turn your microwave on and get arcing or sparks, check the silver card inside your microwave!! Usually located on the right inner side wall or on the ceiling of your microwave. (it might look like metal, but it is not.)

If it has turned brown or black and has a hole or crack in it, this might be all you need to fix it

Some Wave Guide covers are easily attainable and are relatively cheap to purchase and you may want to just go ahead and get the exact one for your model however if this is not the case you can always purchase an inexpensive "Cut To Fit" version. 

Cut to fit Wave Guide Covers are sold either in a 10"x 12" sheet - WAVG10X12 or in 5' x 6" sheets - WAVG5X6 

To use a cut to fit Wave Guide Cover - 

1. Lay old one on top of New Cut To Fit Cover and either "score " the outline around your original one or use a sharp pencil to make an outline 

2. Fot the straight edges I suggest sharp scissors or use an "exacto type knife" with a new blade. (Be careful not to slip and keep fingers out of the way)

3. Cut straight edges first             

4. Then trim rounded corners

5. Check accuracy and replace! 

*if your old one has a "Hole" in it in which a retaining plug goes through you can use a drill bit close to the size of the hole in the original one. 

This doesn't have to be a piece of art so don't panic if its not totally identical to your original. As long as it fits in place and covers the existing hole by around 1/4" all the way around you should be fine and will now have a microwave that doesn't shoot sparks everywhere.