Faulty T4 engine control unit, 2.5 TDI ACV

Hi.
I'm having a problem with the engine control unit (ECU) 074906021 (without any trailing letters) on my T4 with an ACV engine.
The car was running perfectly fine until a few weeks ago, but now a lot of welding work has been done on it.
Recently, the car has been idling much higher than usual, like it has a faulty throttle body. There were also many error codes stored. When I looked up the error codes using VCDS Lite (I only use the Lite version), I found errors in several sensors, including the throttle body, all indicating a short circuit to positive or an open circuit. They cannot be deleted, or they reappear immediately.
Testing the wiring revealed that there were likely no faults in the cable harness.
Disconnecting each of the affected components individually did not resolve the issue (throttle position sensor, mass airflow sensor, intake air temperature sensor, water temperature sensor, fuel temperature sensor, pedal sensor, engine speed sensor).
After studying the wiring diagrams, I realized that all the sensors are connected to the ground provided by the control unit.
I can then, for example, measure 5-6 volts at the connector of the mass airflow sensor (MAF sensor) relative to the battery ground.
Testing with a different control unit revealed that the errors were gone, and the engine idled at the normal speed until it was switched off, as controlled by the WFS (presumably a specific system or function). Similarly, there is no measurable potential difference between the donor mass and the battery ground.
So, I'm assuming that the MSG (presumably a component) is defective. Given the fact that the car was only moved within the workshop and some welding was done, the question arises whether the defect occurred due to the welding, and whether the part can potentially be repaired. Is this error common or well-known?
Due to the WFS, I would like to save the control unit.
Thank you and best regards.

Hello!

It's certainly possible that it happened due to electric welding.

Where I used to learn about cars, back when it was still East Germany (GDR), if the Trabant had an electronic voltage regulator in the alternator or an electronic ignition system, we had to disconnect the battery.

Later, when I was self-employed, I tried to avoid welding on cars as much as possible.
Especially with modern cars, there can sometimes be serious problems if you disconnect the battery.

The welding work focused on the exhaust system.
Most of the time, I would either remove the part or weld it using gas welding.
Or just install something new.

It seems that during the welding process, the conductor track between the sensor ground and the vehicle ground was burned through in the control unit (STG).

I would expect that you should be able to see that clearly after opening the control unit (ECU). However, whether that's the only problem, or if more components have been damaged, you'll find out after repairing the circuit board. I'm hoping the traces are on the top or bottom layer, and not in the two middle trace layers.

For a quick test, you might try bridging the ground cable from the control unit (STG) to the sensor ground. This is not an original solution and is likely to be susceptible to interference. It could definitely be helpful for a quick and dirty test.

Possibly search for the error. I think I've read about this before. With a bit of luck, you might even find a repair manual.

Regards,
Guste.

Hello,

I once had an MSA12 or MSA15, which only worked if the control unit's housing had a ground connection to the vehicle's body (through the mounting screws).
Then it worked perfectly. Possibly a similar case?

Best regards, Rainer.

Hello.
Visually, I don't see any errors, but with that particular arrangement, you can't really see the circuit boards very well.
I also inquired with a company that specializes in repairing engine control units (ECUs), but I only received a cost estimate from them, and they didn't provide any information about whether the issue could be related to the welding.

I also had the idea of simply connecting the donor mass directly to the block, but I have some concerns about whether, in the event of an internal short circuit, it might send more current through the control unit and potentially damage it.
Should the mass connection from the control unit be disconnected in this case?

Thank you and greetings.

"Hi, I'm experiencing the exact same problem with the same vehicle."

Did you have the control unit repaired?