P0237 is Boost pressure circuit A low signal.
Your boost pressure sensor is returning a lower value than expected.
Check all vacuum hoses from vacuum pump to turbo actuator.
Check boost/induction pressure hoses and intercooler for leaks. Liberally dousing it with a soapy solution and then running engine with short, sharp full throttle stabs to build up induction boost would allow any leaks to form bubbles which would be visible.
If all that checks out, check air filter.
Check turbo actuator rod moves all the way.
Diagnostically check maximum MAF reading on a maximum acceleration run. From memory it should be 550-600kg/hr.
If all that checks out ok, clean EGR valve to ensure it closes fully and is not loosing boost pressure.
Finally, if that is ok, get a pressure gauge to manually test boost pressure and compare with that given by ECU via boost pressure sensor. If there is a discrepancy, check out boost pressure sensor.
If none of the above sorts it, suspect worn turbo. Check for turbine shaft movement and look for casing scoring.