ct3- exhaust guard screws broke off and are seized . Penetrating oil for @ 2 weeks. Used a screw impact hand held driver- 3 of 4 broke- only 1 came loose.  Used  Heat- tap and heat- left handed drill- extractor, 2 broke-  Drill and tap I guess.  These screw bodies are below the lip. Any more suggestions?

With the exhaust removed, use a pillar drill to drill them out - it's amazing how much more force you can apply with one of those.

Drill to a max of one mm less than the screw diameter and then tap new threads.  

Drilled and tapped. If I run into bolts as stubborn as these, I may consider tapping earlier.  Thx.

I wonder if no-seize would stand the test of time... and heat..
Sneezles61

Good thought. This is just to hold a guard on and doesnt matter if the screws want to loosen. On an AC unit then lock-tite but in this application just maybe.
Worth a try- Thx