Posted by Alan Kane on August 16, 2009, 6:58 am, in reply to "vw golf tdi"
Hi,
P1556 refers to boost pressure negative deviation, so the boost pressure the ECU is seeing is lower than expected.
First things to check are splits or leaks in the intake system, intercooler etc. Then check the control side of the Turbo, N75 valve and its vac pipes.