Just some food for thought.
I'm a mechanic and been in the game now nearly 20 years
I took a class a few weeks ago that covered EVAP systems.
The reason a check engine light will be lit with a loose fuel Cap on a Gasoline powered vehicle is due to The EVAP system (Charcoal Canister). and the newest computerized sensors detecting a leak of the gas fumes.
Back in the day when computers came out on vehicles, check engine lights were mandated by the EPA should something go wrong with the emissions system. Then the manufacturers added other codes or reasons
for check engine lights to come on in addition to emmisions.
However when it comes to Diesel it is combustable not flamable and should not have a EVAP system because as a general rule diesel does not evaporate
AKA no fumes. but i suppose the manufacturer could have put some kind of sensor that moniters if the cap is tight.
If i'm wrong someone correct me....