With Plane Mechanic Simulator you may be given the problem to 'fix the problem with the guns not firing' on the spitfire.
Fix the Problem with the Guns not Firing
Whilst fixing the Spitfire I was given a timed fix to patch bullet holes, refuel, rearm and 'fix the problem with the guns not firing.
During the timed work I failed to fix the problem with the guns. The next mission I still had the gun issue and I tried everything including a full strip down and rebuild of the weapons system, cleaning the barrels of the guns and still the problem didn't go away. I figured a full strip down of the entire aircraft was needed to see what I was missing.
How to Fix the Problem
To fix the problem with the guns not firing (and this is easier than you think) simply remove the top engine cowling and look at the rear right of the engine block. You will need to click on the starboard cylinder block (assembly) and then remove the air compressor. Then either fix or repair the compressor and reinstall. Once the compressor has been screwed back into place you'll get the completed task message for the problem.