Insufficient Crew
Mech crew are considered to be multifunctional, so they can crew weapons even while tending the engines. The pilot is the only one who must have the ability to do something else while attacking. Otherwise, the full mech crew can be counted toward weapon use.
A mech with less crew than its full allotment will have problems operating. A mech always needs at least one crew member to pilot. If the pilot is able to fire weapons, he may. Elemental-powered, clockwork, and man-powered mechs then need at least half their total crew allotment simply to keep the engines running. This includes things such as turning the cranks on elemental-powered and man-powered ones, and checking the spring torque on clockwork mechs. Any less than half of the total crew and the engines shut down, stopping the whole mech in its tracks.
Once the engine stops, the crew can still fire weapons that don't depend on the engine, such as catapults or javelins, provided they have sufficient crew. Melee weapons usually require the engines to move the limbs they're mounted on, and mechanically powered weapons (such as bore punchers or steam cannons) obviously require the engine. These weapons cannot be fired without sufficient crew.
Crew members occupied in hand-to-hand combat do not count toward the mech's crew (see Boarding a Mech).
