DSTC error not fixed in authorised service after weeks tryin

[borisiak]

Hello, I would need advice/direction on how to solve the problem with the DSTC error message:

Volvo XC60, 2.4 D4 (120kW) AWD, 2014, 174,000 km, automatic

A few months ago, the DSTC warning light came on. The mechanic suggested that it could be a wheel speed sensor issue. They checked, and the sensor was fine.

The problem still occasionally occurred, but the car was drivable.

One day on the highway, the DSTC message appeared again. Then, all the warning lights lit up briefly. While driving, the digital tachometer needle dropped to zero and then returned to the current speed. A power steering message also appeared. After exiting the highway, I slowed down, and the car naturally shifted down to 3rd gear but couldn't shift back up. After turning off and restarting the engine, everything worked again.

The mechanic recommended changing the battery, suspecting it might be the cause. I replaced the battery, but it didn't help. The DSTC warning and tachometer needle drop persisted.

I took the car to an authorized service center, where after three diagnoses, they identified the issue as a power steering pump replacement, which also houses the control unit. The replacement didn't solve the problem.
After driving for 10 minutes, the engine turned off in the middle of an intersection. I restarted it and took the car back to the service.

They then identified a problem in the ring under the steering wheel (steering angle sensor), replaced it, returned the car, but the DSTC error persisted.

I brought the car back to the service, where it stayed for about two weeks. They claimed to have communicated with Volvo in Sweden, followed their instructions, measured various things (likely in the wiring), drove about 300 km, and according to them, the error never occurred. They found no issue and returned the car.

The day after getting it back, the DSTC warning reappeared. I went back to the service, they connected it to diagnostics, saw the error code, sent it to Sweden, but said the car is still drivable.

I left the service, and after a few kilometers, the DSTC warning came on, the car started jerking due to nonsensical down shifting, and the tachometer needle dropped to zero again.

During this period, I noticed that the car sometimes inexplicably starts the radiator fan and roars as if it were 40 degrees celsius outside.

The gearbox was serviced directly at Volvo a few years ago, probably with the replacement of sealing rings because the gearbox jerked when switching from P to D.

Additionally, the alternator was replaced in the car two years ago after failing during a drive, and it was replaced in a non-authorized service.

Thank you for any advice!