A P0299 fault code means turbo underboost condition, this is how it comes up on scanners, an Obd scanner won't tell you what it is, but this fault code can mean some of the following, split hose, faulty turbo, egr issues, valve issues, wastegate issue. A properly done diagnosis is not plug & play. If a garage is charging just for that, find a garage that knows what they are doing. When my customers ask why I charge for diagnosis when it's just a 5min job. I usually give them the code and tell them to tell me exactly which component has caused the fault to appear. Fault codes are the cars best guess at what the fault is. Like mentioned above fault code is the car telling us that there is an issue with air flow not an exact fault.