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Discussion Starter · #1 ·
Hi all,

Fault code came up "17964/P1556 Turbo/super charger boost sensor (A) circuit: control limit not reached" on my mates generic (non-VW) code scanner; my garages scanner didn't come up with any codes when I had it in. I think I've narrowed it down to being a leak, probably from the vacuum hoses (new tandem pump, turbo and N75 valve haven't done anything to help. Turbo boost hoses were renewed only 2 years ago as one was popping out, so I don't think it's those.

Putting it into a German car specialist next week, who are very good apparently. My usual garage did a "smoke test" (I guess testing for vacuum leaks) when I first took it to them for the limp mode problem, but it only showed the smoke leaking from the vacuum pump which they changed and this made no difference.

So my question is, are the vacuum hoses deteriorating (its a 16 year old car...) a pretty common reason for these engines going into limp mode? Is it possible these hose leaks would have gone undetected in a garage but they leak bad when you put your foot down out on the road? Or aside from it possibly being the vacuum hoses being bad, are there other things possibly at fault with the car? It has got progressively worse over the past few months and I don't like taking the car on the motorway now and dread when I have to try and climb a steep hill in it (its embarrassingly bad). Hopefully this new garage will get to the bottom of it because I loved this car before the limp mode problem and I just want it working like it used to again.


Premium Member
9,739 Posts
This is what I have for P1556. Its a common error code search TDI Wiki?

VAG Error Code: 17964/005462

EOBD II Error Code: P1556

Fault Location:
Charge Pressure Control - Negative Deviation

Possible Cause:
Reduced power output.
Limp mode.
Limp mode: What is it, what are the causes, and how to fix it for the Audi and VW TDI diesel turbo.

Boost Pressure too Low.
Hoses/Pipes incorrectly connected, disconnected or leaking.
Charger Pressure Control defective.
Variable Nozzle Turbo (VNT) nozzles stuck.
Solenoid Valve for Boost Pressure Control (N75) defective.

Possible Solutions:
Check Hoses/Pipes to/between Components.
Check Solenoid Valve for Boost Pressure Control (N75).
Check/Clean/Replace Charge Pressure Control.
Check/Clean mechanism for variable nozzles.

As taken from my new Free EOBD II Error Codes software

Discussion Starter · #3 ·
Cheers for the info Keith.

Yeh I just wondered if anyone had any experience of this type of fault being the vacuum lines. Wondered if this was a common cause as I've ruled out a few of the others already.
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