Lack of puff

Having repaired the 620 ti the performance wasn't what you would expect from a 200 bhp saloon. It was somewhat flat, and lacking the urgency that you get with high performance turbo charged motors.
To investigate I fitted a boost gauge, upon opening the throttle boost built up slowly to a max. of 7psi. At first this would point to the electronic boost controller. A device that 'bleeds' boost away from the actuator increasing boost levels above the spring pressure of 7psi.
However this vehicle is fitted with a manual boost controller, so it isn't a problem with the electronic control. Increasing the pressure setting of the MBC didn't increase boost levels.
This points to a problem with the actuator, often with older cars the spring tension weakens, and more pre-load is required to restore correct operation. However too much pre-load results in potentially damaging boost spikes and boost creep at higher rpms.
The correct pre-load can be tested with a simple pump and gauge, increase pre-load to the desired pressure. This is done by adjusting the length of the actuator rod.
After adjustment test drive the vehicle checking boost levels under differing driving loads and speeds.
I had to reduce the settings on the MBC resulting in 12psi of boost (max safe level with std engine) for much of the WOT speed range.

Fault Codes what do they mean?

When you read the codes stored in an ECU you are hoping to find a "silver bullet".
These "bullets" tell you which part to replace or repair, however very few ECU's seem to be loaded with these. Instead the code is an indication of the problem area.
The actual fault still needs to be determined using diagnostic procedures.
Using EOBD the problem area is identified by the code, it then further divides the system into sub systems and finally components/specific operations.

P0420 for example;

P= Powertrain
0= Generic Code
4 = Emissions
20 = Catalyst efficiency below threshold

The prefix letter denotes the system
p = powertrain
B = body
C = Chassis
U = Network

The first number denotes the type of code
0 = Generic (all cars report the same code for a similar fault)
1 = Manufacturer specific ( this can differ from one model to another )

The second number identifies the subsystem
In powertrain;
1 = Fuel or Air
2 = Fuel or Air
3 = Missfire
4 = Emissions
5 = Vehicle or engine speed
6 = Control systems (ECU or output circuit)
7 = Transmission
8 = Transmission
9 = SAE reserved
0 = Fuel pressure control

The final two number numbers identify the setting variable.

Armed with this information it can sometimes help if you get "Blanks" these sound like a real bullets but are a long way short of being a silver bullet.
"unidentified fault codes"