More to diagnostics than just reading a fault code
The rise of the OBD port and cheap diagnostics is a double edged sword. While it does work out cheaper to buy the kit and do your own diagnostics instead of going to a garage, it sometimes requires more advanced equipment to read live data. And some of the cheap code readers don't even pick up all the faults. On my car, a bluetooth reader and Torque didn't pick up P1351 (glow plugs) or P0402 (I've disconnected my EGR valve) whereas the proper Lexia diagnostic kit did.
A good example is the "P" code which reports low pressure at the high pressure pump. On four occasions now I've had customers come to me after a garage and even national breakdown companies have seen this code and said "your high pressure pump is goosed, mate. £400 to replace including parts and labour". And when the fault hasn't been fixed, shoulder shrugging from the garages.
The fault? Simply a neglected fuel filter, either the one in the engine bay, or the one built into the pump in the fuel tank. The proper kit could read fuel flow at the tank pump. And on another it was a poor electrical connection to the fuel pump. The proper diagnostic kit could read the current at the
pump, and it was incredibly weak.
So I'd recommend getting a second opinion from a fellow owner on a car forum for your marque. You'll probably save hundreds.