'02 Jetta ALH. P1556 diagnostic code, + limp mode.
I found conflicting descriptions for P1556, one said that more boost was detected than expected, the other said less boost detected than what was expected. I would go with the less boost option, but I am uncertain. One was from a Haynes manual.
I've replaced several sections of vacuum tubing, and unhooked MAF sensor per online advice "unhook MAF, if performance improves, then replace MAF, as computer will substitute a safe default value in it's absence."
Well the performance did improve slightly unhooked, I can get 5 or 7 PSI boost with the MAF unhooked, no boost with it hooked up. Before and after replacement, no change. Bosch replacement part.
Car has EGR delete, so I swapped the N75 boost control solenoid for the abandoned EGR control solenoid. Still no change! EGR solenoid was of unknown history/condition, however.
Is there a MAP sensor that should be my next likely suspect?
I can hook my MityVac up to see that yes, the vacuum pump is definitely working, and I can also hook my MityVac up to the turbo and start pumping a vacuum on the boost controller, and it immediately starts whining and purring, building boost.
Advice?