Cooling fan running all the time

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The current vehicle customer has been trying to fix this issue since he purchased the vehicle over 6 months ago from the previous owner. The vehicle has been diagnosed in several workshops, including authorised MB. The answers from these workshops ranged from that it is normal for the fan to run without fault, the thermostat has been replaced, and another recommended cooling fan replacement. The MB dealer in Jihlava suggested that the engine ECU software be reflashed. The software reflash wasn't successful because the ECU already had the latest software version installed. The vehicle was returned to the customer with a reply that there was no problem.
In our workshop, we did the following:
Reading fault code memory from all control units – No DTC.
All the temperature sensors – engine, exhaust, intake, air conditioning, transmission – actual values.
Actual readings and test of the fan itself – wiring inspection.
Fan control faultless operation. In the actual values, there is a 45% duty cycle fan activation request (at this point, I am not sure what ECU is requesting the fan activation). The result is that the permanently active fan is cooling the engine coolant and the engine itself.
Interestingly the engine ECU is showing 0 %.
When we simulated an increased engine temperature and A/C pressure, the ECU requested a duty cycle up to 100%.
Without simulation, it's at 45% immediately after the engine starts.
The fan was disconnected to see which ECU was going to complain.
NO ECU IS COMPLAINING!
At this stage, I started to suspect that the engine was chip-tuned.
After investigating with the current and previous owners, I have an answer: the vehicle purchased at an authorised dealer has chip tuning and was provided as a bonus without any written record.

Engine at idle
I disconnected the temperature sensors – the system doesn't activate any DTCs.
I disconnected the airflow sensors – the system doesn't activate any DTCs.
I removed the engine ECU (EDC17CP57) and saved a copy of the software. I then sent the ECU for verification to a person involved with chip tuning.
His reply:
"one idiot have disabled all dtc in file also have made stupid tuning"
He sent me back the same file where he allowed the activation of DTC codes.
"write this file and check dtc , chk updated in file, it looks like B&C tune with DTC patch"
This was followed by something in the line: now the vehicle with activated DTC codes will go into Limp Home mode.
After retrieving the fault code memory, I see many stored DTCs related to temperature sensors. I don't know how long they have been stored in the ECU.
After erasing the DTC memory the cooling fan isn't running at the next engine start.
The following test drive seems OK, and no DTC fault code has been set.
I inform the chip tuning expert about this – his answer:
"you need to drive 4 or 5 gear full throttle or over 160 - 200 km/h",
"it will goes limp mode 100% that why that tuner disabled all dtc"
OK that's clear – during late night driving I confirm that he is right. To my surprise the first temperature sensor that has a problem is the turbocharger temperature sensor. The engine is in Limp Home mode and the cooling fan is ON again. The DTC cannot be erased simply by switching the ignition OFF and ON, and it has to be erased with a scan tool.
The outcome is that it doesn't make sense to continue with further testing, and the engine ECU has to be reflashed with standard software.
After reflashing the standard software and performing a test drive, the car has no fault codes, and the cooling fan works.