Boost Pressure Limit Reached

Good evening.

It's about a 1.9-liter TDI engine with 96 kW (AWX).

Sudden power loss with error code 17964: 'Boost pressure control limit exceeded.' This error cannot be easily cleared. With a bit of luck and some trial and error, it might eventually disappear after some time. The error suggests an issue with the boost pressure control valve or similar components. However, if the mass airflow sensor (MAF) is disconnected and a diode is inserted instead, firstly, this error does not occur, the boost pressure is normal, and the car behaves normally.

What did Zeus say?

'edit: Two LMMs (likely referring to Large Language Models) that I believe are functioning correctly show the same behavior.'

Hello,
LMM failure?
Best regards,
Günther.

lappen wrote:

Sudden power loss with error code 17964: boost pressure regulation limit exceeded. This error cannot be easily cleared. With a bit of luck and some trial and error, it may eventually be deleted after some time.

Um... as long as the error isn't actively occurring while the clearing process is running, it should also be possible to clear it (e.g., when the car is stationary with the engine off), in my opinion.
Whether he will likely be replaced on the next trip is a different matter altogether.

Quote:

This suggests a problem with the boost pressure control valve, etc. However, if the mass airflow sensor (MAF) is disconnected and a diode is instead connected, firstly, the error does not occur, the boost pressure is normal, and the car behaves normally.

Can you make logs of the boost pressure readings available online?
What are typical lambda (air-fuel ratio) and pressure values observed during idle?

Hello Ulf,

'If the engine is not running, you can clear the error code. That's correct. However, as soon as the engine starts, the error reappears. The MAF (Mass Air Flow) sensor value at idle was just below 400mg/stroke when the EGR valve was faulty, which should be okay. Under load, it only reached just above 550mg/stroke. Unfortunately, the MAF sensor works perfectly in another vehicle. I'll need to create logs of the boost pressure tomorrow evening; I won't have time before then. At idle, the ambient air pressure was approximately 940mBar, and the boost pressure was a few mBar below that.'

What's your guess? If it were really the mass airflow sensor (MAF), I don't think this specific error would occur.

I once measured the voltages on the LMM (likely referring to a mass airflow sensor). There are 12V between pin 2 and pin 3, 5V between pin 4 and pin 3, and in idle conditions, approximately 2V between pin 5 and pin 3. Is that also okay?

lappen wrote:

The mass airflow (LMM) value at idle was just below 400 mg/stroke when the EGR system was faulty, which should be okay. Under load, it only reached just above 550 mg/stroke... At idle, I had an ambient pressure of approximately 940 mBar and a boost pressure that was a few mBar lower.

Ah, so it seems the soot limit is being triggered and preventing injection amounts that could really get the turbocharger spinning up.

Quote:

Unfortunately, the mass airflow sensor (MAF) works perfectly fine in another vehicle.

... and another diagnostic tool shows the same error on the problem vehicle, as you mentioned above.
Then the cause is likely related to the mass airflow sensor (MAF) connector, specifically either a wiring harness issue or a problem with the main system control module (MSG).
Have you measured the voltages when the mass airflow sensor is faulty?
If so, only the signal cable to the MSG, the connector to the MSG, and the MSG itself should be considered (e.g., a faulty ADC in the MAF sensor input).

Next, I would try replacing the MSG (Message Security Gateway).

Hi Ulf,

I'm not entirely convinced it's the connector or the signal line, because we attached the connector for the mass airflow sensor (MAF) with the diode 'on board' in exactly the position where it would normally be connected to the MAF. Also, the ADC (Analog-to-Digital Converter) within the control unit (ECU) shouldn't be the problem, since it works with the diode and I'm getting a reading of almost 1250mg/stroke for the maximum possible air mass. If there were a defect in the signal path or the ECU itself, then using the diode wouldn't solve the issue.
I measured the voltages, except for the mass airflow sensor (MAF) signal which was measured with the MAF disconnected, while the MAF was still plugged in. Sorry, that was my mistake. I will repeat the measurements differently tonight.

lappen wrote:

The ADC in the ECU should also not be a problem, since it works with the diode and I am getting almost the maximum possible air mass reading of approximately 1250 mg/stroke.

If there were a defect in the signal path or the ECU itself, then the diode wouldn't be working correctly.

Quote:

Yes, that's hard to argue against – unless the ADC has a completely distorted characteristic curve due to a defect, although in that case, the lower limit (LL) and maximum value would presumably still be displayed correctly. But whether something like that is even possible...??

The voltages, except for the LMM signal, which was measured at idle, were measured with the LMM disconnected. Sorry, it's my mistake. I will repeat the measurements differently tonight.
Then let's hope for "incorrect" values when the system is stressed by the mass airflow sensor (MAF). And since we're already dealing with such exotic things, also check the voltage of the sensor ground at the MAF against vehicle ground }.

Then let's hope for 'incorrect' values when the system is stressed by the mass airflow sensor (MAF). And since we're already talking about such exotic things, also check the voltage of the sensor ground at the MAF against vehicle ground .


I don't want to sound like I know everything, but why would the mass airflow sensor (MAF) pull down the voltage? The 5V for the MAF signal probably shouldn't be responsible for that, since the diode should still provide the full air mass reading. It could potentially be the 12V supply voltage for the MAF, but I doubt it, because the air mass seems correct at idle, and the MAF also reacts during driving, even if only slightly.

It's quite mysterious, the whole thing.

lappen wrote:

I don't want to sound like a know-it-all, but why would the mass airflow sensor (MAF) pull down the voltages? The 5V for the MAF signal probably shouldn't be responsible, since the diode should also provide the full air mass. It could potentially be the 12V used as the operating voltage for the MAF, but I doubt it, because the air mass seems correct at least when idling, and the MAF still reacts during driving, although to a lesser extent...

Depending on the internal resistance and load, every voltage collapses .

If the internal resistance of the 5 or 12V system is increased due to a defect (in the control unit or wiring harness), the load caused by the mass airflow sensor (MAF) can lead to a voltage drop, while direct feedback via the diode only causes a noticeable voltage drop in the high-impedance measurement input of the control unit (or even measuring the unloaded voltages) will not yet cause an obvious voltage drop.

It might be a rather unusual cause, but the symptoms are certainly something new.

"In my opinion, a series resistor placed on the 12V wire, which supplies power to the mass airflow sensor (MAF), would be a good fit for the observed symptoms."
For idling conditions, the current might still be sufficient, but at higher airflow rates, the mass airflow sensor (MAF) cools down and reports insufficient flow because not enough heating power is supplied – as far as I know, the MAF voltage is ultimately only a measure of the heating power.

This also explains the usual insidious "hide-and-seek" game that these kinds of errors play.

1. When measuring the voltage without a load (with the mass airflow sensor disconnected), it appears completely normal.

2. Even at idle, no significant drop in voltage could be detected.

3. To catch the error in the act, one would need to measure the voltage while driving when power is demanded but nothing happens.
With a bit of luck, it might also work if you rev the engine while it's stationary: at 4000 rpm, an airflow approximately in the range of 2000 rpm is reached with full boost pressure, and there should be a noticeable drop in the 12V voltage.

I'm curious to see what the real cause is.
Because when you predict insidious errors, the true cause is often found elsewhere, according to Murphy's Law...

Okay, so I've measured the voltages at idle again with the mass airflow sensor (MAF) connected, and they are fine. However, it's proving difficult to measure the voltages while driving or during high RPMs when stationary, so I can't say for sure what's happening there. Since the 5V is obviously present and appears stable even with a diode check, it could be that only the 12V supply is the issue. Later, I'll try applying an external 12V to see if that makes a difference. If nothing changes, then I'm at a loss. But if it does improve, then the 12V amplifier in the control unit (ECU) likely has a problem. Or am I missing something?

PS: It definitely isn't the mass airflow sensor (MAF). I had it replaced by a certified mechanic today, and it was guaranteed to be new. I also tested the replacement MAF in another vehicle and confirmed that it works correctly.

lappen wrote:

Since the 5V is obviously present and apparently stable with a diode, it could only be the 12V that's causing the issue. I will try applying an external 12V later. If nothing improves, then I don't know what else to do. If it does improve, then the 12V amplifier in the control unit (STG) probably has a problem. Or am I missing something?

A small one:
The 12V power supply for the mass airflow sensor (MAF) in the (few) SLP models I'm familiar with doesn't come from the main fuse box (MSG), but rather through terminal 15 or the infamous relay 109.
Apparently, there is no 12V regulator available in the MSG.

For example, in the Polo 9N, there's a mini-fuse located between terminal 15 and the mass airflow sensor (MAF), which can be a potential source of voltage drops that might easily go unnoticed.

Quote:

However, since the 5V are obviously present and apparently stable with a diode

, ... that doesn't necessarily mean anything.
With an input impedance of 10 kOhms for the LMM signal, the 5V output of the MSG must be able to stably supply only approximately 0.5 mA during the diode trick procedure.
However, if a mass airflow sensor (LMM) is connected, it will likely draw significantly more power. And whether the 5V supply remains stable under that load is not necessarily guaranteed (especially when considering the possibility of a faulty 5V regulator).

Ulf,

You are right. I just connected the battery voltage to pin 2. Same result. Only, when connecting it, some things in the dashboard lit up without turning the ignition key, and the engine didn't turn off completely when turning the ignition key from 'on' to 'off'...

'Then it's probably due to a failing 5V line. But how can I test that effectively right now? I don't have much technical equipment available here. Fortunately, I'll have more resources this weekend. I have a small battery power supply with a 6V output. Perhaps I could put a silicon rectifier diode in the positive lead and then connect it to pin 4 and ground to the vehicle's chassis. Do you think that would work?'

lappen wrote:

Then it's probably due to a collapsing 5V line. But how can I test that effectively right now? I don't have much technical equipment available here. First of all, I'll be better equipped over the weekend. I have a small battery power supply. It also has a 6V output. Maybe I could put a Si rectifier diode in the power line and then connect it to pin 4 and ground to the vehicle's ground. Do you think that will work?

In principle, yes, the mass airflow sensor should be able to handle approximately 5.4V instead of 5.0V - provided that your power supply really delivers a stabilized 6.0V .

But why don't you simply tap into the 5V wire while the mass airflow sensor and motor are connected?
That would be simpler for now.
Unfortunately, I don't know whether the power consumption of the mass flow meter also increases significantly with the airflow when powered by the 5V supply.
If the 5V are present at the motor control unit (LL), I would immediately proceed by connecting the 5V to the mass airflow sensor and , tapping into the signal input at the engine control module (MSG), and running both as temporary measurement wires into the interior.
Then get in the car, briefly rev the engine to around 4500 rpm, and first observe the voltage at the MAF sensor input (using an analog multimeter). In the range of 4000 rpm, the boost pressure usually increases as well, so the MAF sensor voltage should then rise significantly above 4V.
If that happens, the MSG's LMM input is faulty (even though the diode is working).
If the signal voltage is lower than expected, the 5V should also be measured using the same procedure.

If the 5V supply fails, a new main system board (MSB) will likely be needed.
And in the end, the error even survives because everything is related to a transition resistance in the 5V line or the ground line (I don't know what happens with ground faults in the MAF sensor) .

Let's switch gears for a moment: Could it perhaps be related to the boost pressure sensor?

But wouldn't everything be perfect with the 'diode-based LMM' either?

Okay, I missed that, sorry.