The presence or absence of fault codes proves nothing. The ECM will log a fault code if it receives information from a component which is outside of its programmed parameters. The engine could be running very poorly, but the ECM would not know that unless something told it.

An EOBD (known as OBD II in North America) scantool is only capable of reading engine emission related data. They cannot read manufacturer specific data or any other controller on the vehicle. This will be why you are not seeing any fault codes. The likelihood is they're present, you just cannot access them with the equipment you have.

You need to go to someone who does have a suitable scantool for your vehicle. Anything you do will be mainly guesswork which in the long term will probably prove more costly to you than taking it to a garage in the first place.