In a jap spec, other than vacum lines, what is the most common fault for going into limp mode?

 

Vacuum lines themselves don't really cause "limp mode". If loose, missing or mis-routed; they will make the car run bad but that won't necessarily be "limp mode".

Limp mode is when the ECU detects that an electrical component it deems as important is either:

* missing
* sending a signal to the engine that something is bad