TDI Cold Start: Emergency Mode Troubleshooting

Hello,

I'm new to this forum and not an experienced mechanic, but I'm going to ask a question anyway.

"For some time now, it has become more frequent that my A2 1.4 TDI (manufactured in 10/2003) goes into limp mode after a cold start. This always happens when I start the engine according to the owner's manual, i.e., waiting until the preheating indicator goes out and then starting it."
"The error code that appears each time the fault memory is read is: 'Implausible relationship between altitude sensor/boost pressure sensor signal'."
The friendly technician has already replaced the pressure sensor for the intercooler, the turbocharger, and the mass airflow sensor, but the error still persists.

Has anyone perhaps encountered the same problem before, or does anyone have an idea what might be broken? I'm really desperate; the car has been in the workshop for at least a week now, and it hasn't helped.


Thank you in advance for your answers.

Best regards,
Michael

Michael_A2 wrote:

"The error code that always appears when reading the error memory is: 'Implausible relationship between altitude sensor/boost pressure sensor signal'." The friendly mechanic has already replaced the pressure sensor for the intercooler, the turbocharger, and the mass airflow sensor, but the error still occurs.

Typical workshop. But at least they've already replaced one possible culprit – the boost pressure sensor (which you refer to as the pressure sensor for the intercooler).

More recent MSG (Multi-Sensor Gateway) devices continuously compare the pressure readings from the LD (Laser Distance) sensor and the ambient pressure sensor within the MSG. In the event of significant discrepancies, they switch to a "something is wrong" mode, which is a fail-safe mode (e.g., to prevent or make it more difficult to generate false LD signals).

To fully understand the problem, instead of performing the standard procedure (querying the error memory), those with limited technical skills would only need to access the MWB 10 and compare the values of the two pressure sensors. Normal values are around 1000 mbar or slightly below (depending on the weather and altitude).
If they were unsure which pump was malfunctioning, they could have quickly connected to a different vehicle for a short time.

If you've already paid for the loader and LMM (likely referring to specific parts or services), I would demand a full refund – or would you also let them sell you a set of new tires just because the brake pad wear indicator is on?

Hi,

Just check the value in measurement block 10 for the ignition system in the engine control unit.
It shows the pressure measured by the ambient pressure sensor in the control unit, and the current actual boost pressure. Both values should be approximately the same.
A quick look at a barometer will also tell you which measurement is incorrect.

Best regards, Rainer.

Hello,

First of all, thank you for the answers. There are actually knowledgeable people around here.
When I went back to the workshop today, they finally, after the fifth time, came up with the idea of reading out the pressure values from the MWB 10. Since the turbocharger pressure sensor has already been replaced twice, it can only be the ambient pressure sensor that is defective. Ulf says that the ambient pressure sensor is integrated into the MSG (Main System Module). Do we now have to replace the entire MSG, or can the sensor be replaced individually?
According to the workshop, the MSG (presumably referring to a specific part or service) costs around 800 euros, which would be quite annoying.

P.S.: By the way, I didn't have to pay for the LMM (mass airflow sensor) or the turbocharger because I took out an extended warranty on the used car before the original warranty expired. So, it's not really about the money for me. I'm just tired of constantly having to take my car, which is only two years old, to the workshop, only to find out that it still doesn't work perfectly.

Michael_A2 wrote:

Ulf writes that the ambient pressure sensor is integrated into the MSG. Does this mean that the entire MSG needs to be replaced, or can the sensor be replaced individually?

Quote:

According to the factory instructions: yes. Of course, electronics professionals can also replace the height sensor in the MSG.

According to the workshop, the MSG costs around 800€, which would be quite annoying. . . . By the way, I didn't have to pay for the MAF sensor and the turbocharger because I took out an extended warranty on the used car before the original warranty expired. Then you also don't have to worry about a new MSG }