If “30” appears in the display it means that the anti flood /overfill device has activated.
The instruction book simply advises to call an engineer, which is probably the best course of action.
What this fault code means is that the dishwasher is leaking (or at least a device to detect leaks has triggered). In the base of a dishwasher there is usually a micro switch and float mechanism, which activates if water gets into the base.
The trouble with dishwashers is that they are very difficult to work on. There’s no back panel to take off so you can’t have a quick look inside. There is a lid to take off but it only reveals a sealed stainless steel tank and a few hoses.
However, it might be worth a quick look in case you can see evidence of a leak.
Other than that, in order to get to see inside the base of the dishwasher you would need to start stripping it down removing the kick strip at the front.
It may even be possible that these won’t come off without taking the side panels off, and in turn the side panels might not come off until a back panel or the lid is off.
So basically, the chances are there is water in the base of the dishwasher caused by something leaking.
Unless you are prepared to strip it down to investigate, best get an engineer.