TDI Golf 3 (AFN): Limp Mode Again?

Yesterday, I experienced the emergency mode twice on the highway again, after a period of 2 months without any issues. Here's what happened: I was driving uphill on a three-lane highway when a Touareg suddenly pulled into the middle lane for no apparent reason. I couldn't move to the left. So, I braked, and for some unknown reason, I shifted back into fourth gear. When I tried to accelerate, the car went into emergency mode. I stopped at the next rest area, turned the engine off and on again, and two minutes later, while driving uphill again at a low RPM, the same thing happened.

After that, I had a peaceful rest of the journey, about 150 km, and both the top speed and acceleration were as expected.

The error codes retrieved from the diagnostic system were as follows:

Intake manifold pressure control differential.
Intake manifold pressure sensor short circuit to positive.
Intermittent control unit failure.

The "ECU intermittently defective" error is triggered every time I give it full throttle. However, the car continues to drive normally and doesn't go into limp mode. I observed this during a test drive. I cleared the error memory beforehand, and when I gave it full throttle for several seconds on the first attempt, the "ECU intermittently defective" error was set without affecting the driving behavior.

What could that be? Has the control unit finally failed?

The car is a 1997 TDI with a 110 hp AFN engine. The mass airflow sensor (MAF) and pressure transducer have been replaced. The variable turbocharger (VTG) has already been repaired. The ECU numbers are: 028 906 021 GH, and the Bosch part number is 0281001652.

Quote:

What could that be? Has the control unit failed after all?

"Recently, there have been isolated instances of cable breaks occurring at the transition point between the motor and the body, specifically between the starter and the battery box. Please monitor the vacuum pressure for any irregularities or spikes, especially if someone is simultaneously moving the wires in that area."

Today I did a diagnostic test using a laptop, VAG-COM software, and an OBD cable, and everything was completely normal, even though I intentionally tried to trigger it and recreate the situation. Maximum speed, acceleration, power – all were as expected…

Recently, there have been isolated instances of cable breaks occurring at the transition point between the motor and the body, specifically between the starter and the battery box.

Oh yeah... that's really bad!! I already replaced that €60 cable harness.
Aber eigentlich fällt bei so einem defekt die gesamte Elektronik aus. So -> dark.

Best regards.
p

"I haven't checked for any cable breaks yet – the car is running normally again. The strange thing is that on every trip, whenever you accelerate fully – whether it's to get onto the highway or to overtake – the error message 'ECU intermittently defective' appears. However, you don't notice anything unusual while driving..."

Quote:

Aber eigentlich fällt bei so einem defekt die gesamte Elektronik aus. So -> dark.

I have encountered this error twice already this year.

Both were Skoda Octavias, one from 1998 with a broken cable in the power supply for the Hall sensor, and the other was an Octavia 1Z with engine BJB from 2004 with a broken wire for the intake manifold pressure sensor. In this case, when the sensor failed intermittently, the ECU sporadically set a substitute value of approximately 2450 bar, which naturally prevented the build-up of boost pressure. Additionally, in addition to the intake manifold pressure sensor, the error "altitude signal" was also incorrectly flagged as implausible , although plausible values were displayed for this parameter.

In the Volkswagen Golf IV, vibrations can cause chafing of the cable insulation on the wires located within the rigid conduits.

These or similar errors are very insidious and can cause the most bizarre problems.