A6 2.5 TDI: 1000+ Common Problems & Fixes

Hi, so I'm having a lot of problems with my TDI. It would be great if someone could help me out a little.

Audi A6 2.5 TDI 115 hp, manufactured in December 1994 (C4), 85,000 km

So far, the car has exhibited the following symptoms:

1. No preheating during cold starts (the preheating indicator light briefly illuminates and then goes off, both in summer and winter), and therefore, it only starts after a long cranking process, if at all.
I replaced the upper thermostat switch, but nothing has changed.
'So, I disconnected the cable from the glow plug switch, and lo and behold: it at least glows now, but only for about 20 seconds or so. That's definitely too long. But at least it starts now. However, in cold outside temperatures, the engine takes a very long time to warm up. I also replaced the glow plug relay, but that didn't help.'

'As soon as I drive for 5 minutes without braking, the preheating indicator (which is also supposed to indicate electronic problems) starts to blink. If you then press the brake pedal, it goes out for the next 5 minutes. So, I replaced the brake light switch, but nothing changed. Adjusting it also didn't help.'

3. At higher speeds, approximately 120-160 km/h, there is occasional, strong jolting (the engine briefly cuts out).

4. Insufficient power. The top speed is only about 160 km/h, and the car feels underpowered.

Then I wanted to connect the car to the diagnostic device in the workshop and noticed that a fuse near the control unit had blown. I replaced it, but now the car won't start at all, and the preheating indicator is constantly flashing and won't go away. So, in order to be able to drive again, I haven't put any fuse in at all. And since then, symptom number 3 has disappeared; it no longer jerks. However, it still doesn't reach speeds above 160 km/h.


According to the device's error diagnosis, as reported by the workshop:

3 mistakes.

-00741- Brake pedal monitoring, implausible signal, intermittent error.

-01242- Power stage in the control unit, open circuit / short circuit after positive terminal, intermittent fault.

-00542- Needle stroke sensor for G80, input open, intermittent error.
'Die Sonne scheint hell, und die Vögel zwitschern fröhlich. Es ist ein wunderschöner Tag, um spazieren zu gehen oder ein Picknick im Park zu machen. Viele Menschen genießen das gute Wetter und verbringen Zeit im Freien.'

According to the workshop, I could replace everything (ECU, diesel pump, mass airflow sensor, wiring harness, etc.), and the problems might still be there. If I knew exactly what was broken, I wouldn't hesitate to replace it, but the prices for the parts mentioned above are very high, and I don't want to replace half the car without any guarantee that it will fix the issue.


Does anyone have experience with the errors mentioned above, or know how to fix them?

Hi,
First, I would check all the cable bundles for short circuits; perhaps there's a frayed wire somewhere. And if the control unit itself is identified as the problem (due to a short circuit), I would start by examining the control unit closely. Perhaps a good tuning shop, like DigiTec, could take a look and help you diagnose the issue.

Because if the control unit is defective, it's possible that other errors will result from this defect.