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p1556 Code Low Boost

58253 Views 14 Replies 6 Participants Last post by  pbdr
I just bought a 2002 TDI Jetta ALH with turbo issues for a good price. Before I bought the car I had a mechanic drive the car and he said the turbo was bad. It feels like the turbo is never engaged. I do not have a vag com, but the auto scanner returned a p1556 code which I believe translates into boost deviation pressure negative. I replaced all of the vacuum lines, the boost pressure sensor, the actuator, and cleaned the intake and egr valve. The piece on the turbo that the actuator rod attaches to seems to move freely. I am going to check the N75 valve later this week. I pulled the electrical connection off of the MAF sensor and there was no change in the way the car ran. I do not think this is the problem, but I dont know. Should I pull the turbo off and clean it or just buy a new turbo:ugh?
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Welcome to the forum. welcometomyturbodies

VAG Error Code: 17964

EOBD II Error Code: P1556

Fault Location:
Turbocharger (TC) pressure - control limit not reached

Possible Cause:
Turbocharger (TC) wastegate regulating valve.
Intake leak.
Turbocharger (TC) defective.

As taken from my Free EOBD II Error Codes software
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If there's no change in power with a disconnected MAF it means that you're not getting air. It could be the turbo or something else. You could just buy another MAF sensor but I would suggest testing it by datalogging the sensor reading and seeing if the actual value it's putting out matches or is at least close to the value the ECU specifies.

The actuator was definitely bad. I think I noticed a bit more power after disconnecting the MAF sensor so I went ahead and ordered a new Bosch MAF sensor for $95. I looked into buying the VAG COM and software, but it seems woefully insufficient in terms of instructions on how to troubleshoot problems and interpret data for someone like me who has no experience with this software. Is there a book that comes with the software or a step by step guide on how to retrieve and interpret the data?
Thanks for the help.
Look through the FAQ for step by step instructions on how to use it and troubleshoot. There are label files which tell you the specifications but you have to know how to use the tool.
Is there a book that comes with the software or a step by step guide on how to retrieve and interpret the data?
Do a search for VCDS or what ever in here. ;)

I had same problem

I recently had the same problem, and it turned out to the the EGR, intake manifold, dirty turbo, and the small check valve just below the N75 was bad. How many miles does the car have?
After replacing the actuator, cleaning the intake and EGR valve, and replacing all vacuum lines the turbo is finally working better, but is not a 100% and I am still getting the P1556 low boost pressure deviation code and an excessive flow EGR valve code. Will a bad EGR valve cause low boost pressure? The car has 145,000 miles on it. I am going to do a boost pressure test. Thanks for all of the help from this site. Great site.:bowdown
I'm not sure how VCDS works but isn't there something in there to resets all error codes after you have done a repair so you start from scratch? ;)

I just dealt with that code on my car and replacing the VNT actuator did it for me.

One simple thing, is it getting air? Could be a seriously clogged filter or intatke maybe? The boost test might give you a little more info.

Best of luck.
Yes I replaced the air filter and cleaned the snowscreen also, but replacing the actuator made a world of difference. I am still getting the low boost deviation code P1556 and an EGR excessive flow code, but the car runs much better up until 75 mph then it feels like I lose all power when I hit the accelerator. Still planning on doing the boost pressure test. Thanks. :eek:
Problem Solved

I bought a mighty vac and went through the vacuum lines looking for vacuum leaks and discovered that the EGR diaphragm was bad (would not hold vacuum). After replacing the EGR valve, the turbo works fine. The previous owner said the EGR valve might be bad and I believe the EGR vacuum leak caused the turbo to not work properly, eventually allowing the actuator to rust and fail. Nice to have the turbo back and I hope I dont get a ticket. Thought this information might help someone else fix their limp mode issues. Thanks for all of the help and feedback. :thumbsup
Thanks for the follow up. Good to hear it was fixed and now you have another tool at your disposal!
Any news? Solution found??? I'd like to hear how it's going...

Sorry, the "problem solved" was only noticed after I posted....look at all pages of the post! Looks good, now let me try!
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