Hi

You dont give much background info of the vehicle fault / test result so far. or for that matter on your own experience so it is difficult for members to know where to pitch replies.

I don't know the answer to your fault but,'No fault codes' can be as much a clue as some fault codes are.

No fault codes would suggest it is not a electrical circuit fault or a component that is closely monitored. So have a look at the system in front of you with that in mind and then monitor relevant live data to look for data that is out of the normal.

You may also need to physically measure areas of concern eg fuel pressure, manifold & exhaust pressures etc.

cheers Aidan