Hey everyone,
I’m having a weird issue with my 2016 Volkswagen Passat TDI — the power suddenly drops while driving, and I keep getting code P029900: “Boost pressure regulation: control range not reached.”
Here’s what I’ve done and checked so far:
The diverter (bypass) valve seems to be working fine
Vacuum supply is good
Vacuum lines look intact, no visible cracks or leaks
I already replaced the boost control solenoid (N75-style valve), but the problem is still there
I did a vacuum test on the bypass valve — at first it builds vacuum properly on startup. But after a few presses on the gas pedal, the vacuum drops off completely after a few seconds, and the P029900 code comes back.
It feels like something is cutting vacuum control after a few seconds of running.
Any ideas what could be causing this? Could it be the vacuum pump? A leak I’m not seeing? ECU issue?
Appreciate any help!
Sudden Loss of Power – Code P029900 on 2016 VW Passat TDI (Boost Pressure Regulation Issue)
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I’ve had a similar issue on my Audi A4 TDI - same P029900 code. I used InCarDoc to monitor live boost and vacuum readings, and noticed the vacuum would drop right when the ECU was trying to build boost under load. Turned out to be a weak vacuum pump.
Try using InCarDoc while driving to log the vacuum behavior - if it consistently drops after throttle input, the pump might not be holding pressure, even if the lines and valve look fine.
Good move checking the vacuum lines and replacing the solenoid - that's usually where I’d start too. When I had this on my Golf TDI, InCarDoc helped spot inconsistent turbo actuator behavior.
Watch the "Boost Pressure (Desired vs. Actual)" and "Turbo Control Duty Cycle" in real-time. If you see the desired boost going up but the actual doesn't follow, that confirms it's a regulation issue. Could still be the actuator sticking or a software/ECU logic glitch.
I had a really similar issue on my 2015 Audi A4 2.0 TDI — same P029900 code and the same “vacuum disappears after a few seconds” behavior. I hooked up InCarDoc to watch live boost and vacuum data, and it was clear that the vacuum dropped off as soon as the ECU decided to cut control to the turbo actuator.
In my case, it wasn’t the vacuum pump at all — it turned out to be a small split in the vacuum line hidden right under the heat shield near the turbo. The line looked fine at first glance, but when it got warm, the crack opened and the vacuum dropped instantly.
If you can, try logging with InCarDoc while flexing the lines by hand — sometimes you can see the drop happen in real time. Saved me from swapping a perfectly good pump.
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