[AntonLargiader]

If there has never been a fault code, that tells you that the fault is somewhere that the BMS cannot see. That's useful information. For instance, it doesn't know if the coils are working (it cannot see the secondary windings) but it does know if the injectors are disconnected. It can probably see a dead stepper motor but maybe not a stuck plunger.

If you have a scanner (like a GS-911) you could observe some of the outputs such as injection time. That will tell you if the engine is compensating for something when you get the idle problem. Whatever is happening WILL be visible by observing the right output. I just don't have a list of possible fault codes. For instance, if the engine becomes too out-of-balance left to right, will it register some kind of 'excessive compensation required' code? I don't think so. But watching the injection time and stepper position you will still see this.

Sounds like a fun project if the tools are available. Pretty frustrating if they're not, I can tell.