transit 350 62 plate poobd fault code

[Ian fryer]

Our 2013 Ford transit had a second hand engine replacement.
About a week after having it back it was making a high pitched squeal so it went back to the garage which they said was the turbo casing. They put our original turbo back on it and it stopped the noise, but in doing so, somehow created a fault light to come on when the engine reaches 3000 revs and then it goes into limp mode. When you turn it off and back on again the light is off and it’s out of limp mode.
Nearly 6 months on the garage who did the engine replacement cannot find the fault. It has been looked at by furthers technicians and at other garages and no one can find the fault or what it is. All garages have exhausted every option they can think of and can’t think what else to try.
The fault code is P00BD. Mass air flow.

Any advice on next steps would be much appreciated. We rely on the van for income and are out if pocket for the engine replacement and further exploratory works too. List below and in photo of the main things that have been tried, though other pipes and bits & pieces have also been switched and replaced.

1. second hand engine replacement 
2. Trying to sort fault code P00BD. Mass airflow performance flow too high.
3. Replaced second hand parts off low loader that worked on other vehicle; mass air flow sensor, alternator, EGR valve, turbo, intercooler
4. Further works and investigations carried out by another garage are detailed in type on photo.