With most Project Ems i have rebuilt have either stuffed score reel or stepper unit and without these working correctly the game just wont start. So on all ems this is the first thing i do to attempt to get it back to life.  Obviously yours will still be hanging from your machine unless you have enough patience to mark and unsolder all the wires (STUFF THAT) 
 There are a few types of steppers

1. Continuous which has one coil and steps in one direction   (Used mainly on features bonus scores etc)
2. Step up / Step down which has 2 coils one to step it up and one to step it down   (Mainly used for bonus scores etc)
3 Step up  / Reset which also has 2 coils ine to step up and one to reset to the start position  (used for player units and ball count etc)

If a game wont boot id look at steppers with two coils first as features shouldn't stop the game from starting.

Im using a Continuous stepper for the Demo but same rules apply for the reset or step down section as the step up section

Here are some photos of the stepper arms these guys do most of the work.

STEP 1 first thing i do is blow the unit out with compressed air or attack it with a dry paint brush to remove any unwanted   
            dust  bugs etc.

STEP 2 any leftover grease hard dirt etc i clean with a toothbrush dipped in metho preferably not your own especially around
            the pivot point

The second photo shows the build up grease that blowing and paint brushing didn't remove third show what a quick hit with motho on a toothbrush can remove

ty for posting this, I am back in the land of the EM (await news of my next purchase  ) and will need to know this stuff.
 

STEP 4 If these don't move freely and they are metal on metal a small drop of machine oil should do the trick but dont lube any  
            non metal on metal parts these shouldn't need any oiL

      In the photo show where lube is ok and where it is not some steppers may be different but this one has a nylon bush over
      the shaft so no lube is needed

Great work so far Daniel .
 For those reading this not familiar with EM's it can't be stressed enough how important it is to have all steppers moving freely .
I have found it is well worth the effort to take the time to overhaul all stepper units in any game you purchase (unless you know the machines maintenance history) because it is almost a sure bet they have never been cleaned since the machine was built.

Step 7   Clean the plates with metho on a toothbrush then a rag once that dries steel a Scotch / Jex pad thingy from the Kitchen
              i usually cut these into finger wide strips so i can see whats going on while im using them but you can use the whole thing if you really want. Then gently rub all of the Brass buttons or copper PCBS untill they are nice and bright \ shiny

The first photo shows how the unit started
The second photo shows half the plate cleaned and the other as it was found (this step is not needed only done to compare)
The third is how it should look

STEP 9  Now on the steppers with the spring fingers jsut double check that these move in and out freely  if they dont clean inside as best you can until they do

Step 11   Now line up your marks you made earlier (You did mark it right?  ) and screw or bolt back into place