Do Mechanicus continue to work on planets that turn traitor?

So my understanding is that the Mechanicus are wide spread and maintain all day to day tech of the Imperium. So what does that mean for Mechanicus cult members when a planet such as Vraks or Krieg turns traitor? They are loyal to the Omnisiah not imperial cult priests like Xaphan or Planetary Lords. Wouldn't all the Imperium have to do is ask the Fabricator-General to order them to shut off a Hive World's reactors instantly ending any potential heresy? What are the heretics going to do? kill the techs? then who would keep all their equipment running while they fight against the Imperium loyalists?