ECU for VW Passat 35i 1Z: Replacement, Faults, Compatibility

Hello, Dieselschraube community.
My Passat TDI 1Z, manufactured in 1994, now has approximately 195,000 kilometers on the odometer, and it has been with me for the past 10,000 kilometers and 3 months.
I bought it in relatively good condition. The pre-glow LED wasn't working, but the starting behavior was normal. The previous owner said, "The LED is broken, but it always starts perfectly."
"Since I've had it, the engine hasn't been reaching its operating temperature. The fuel consumption has always been 5.5 liters (with the air conditioning always on, usually with 3-4 people in the car, mostly highway driving). The thermostat was replaced by the dealer before the sale (along with the timing belt, brakes, etc.)."

Suddenly: 6.7 liters per 100 kilometers fuel consumption. I went to the workshop because I wanted to get a few other things fixed as well.
Result: Thermostat faulty and replaced, 2 glow plugs faulty and replaced.
"However, the water temperature doesn't exceed 70 degrees Celsius, and the preheating LED doesn't illuminate. Additionally, there's an issue with the fan control system related to the climate control unit."

Now my mechanic has arrived, and it's likely coming from the control unit.
All sensor readings are being received correctly by the control unit. It only briefly illuminates a "failsafe" indicator. It doesn't reach operating temperature consistently while driving, but the temperature sometimes fluctuates significantly. Additionally, the control of the radiator fans isn't working as it should.

Now, I started looking for a used engine control unit.

Mine has the Bosch part number "028906021B".

org. and the new one costs an incredible €1100.
My mechanic made some phone calls, but so far, he hasn't come across any like it in his network of car recyclers and parts suppliers.
I was browsing eBay the other day and came across several engine control units (ECUs) that had the same part number, except for the last letter. Some were from Passats, while others were from other 1Z engines (Audi 80, Golf).
Which engine control units (ECUs) are compatible? Are any ECUs compatible at all? What should I pay attention to when buying used ECUs? Can the ECUs be reset to factory settings if the injection timings, etc. have been altered by tuners?
In a quote, I read "without transponder." What can I understand by that?

(If it's relevant; I have a Climatronic system without the Eco function...)

I've also tried to learn more about the subject, but I haven't really gotten very far. I hope you understand my problem and can help me out.

Best regards,
Sven.



[/i]

Hello Sven,

Has a new coolant temperature sensor already been installed?
If the new thermostat is functioning as it should and the temperature readings fluctuate, then the only remaining possibility is a faulty temperature sensor, right?

The engine control unit is usually quite robust.

If the temperature sensor reports excessively high temperature values, the engine control unit will not activate the glow plugs. Often, faulty sensors will also "jump" randomly between warm and cold readings.

Best regards, Rainer.

I've already posted a lot about 1Zs that don't heat up properly, including posts with graphics and other visuals.

My temperature sensor wasn't really defective, but I replaced it anyway.
The thermostat as well.

... back then ...

m;