citroen c5 - AFTER THE AUTOMATIC GEARBOX REPLACED, THE CAR STILL DOESN'T STA

Hello, I have a problem with the automatic transmission in Citroen C5 2.2 hdi 133 KM from 2002, the symbol of the gearbox is 20HZ20. 4 gears. The problem is that I replaced the gearbox in my car, because the previous one stopped shifting gears. Sometimes it drove one meter forward, backward and again refused to obey. The repair cost was too expensive, so I decided to buy another used one. The seller assured me that it is 100% functional, dismantled from a moving car. The exchange was performed by a workshop dealing with the exchange of crates. Unfortunately, after the replacement, the car still does not start, despite the fact that the engine starts immediately. At the moment of starting the engine you can hear a characteristic quite loud whistle, which was not the case before. The torque converter was also replaced. When I switch to manual, the light marked with the number 2 turns on when I throw a 2 and it immediately returns to position 1. exactly as before replacement. The mechanic claims that it is a problem with the gearbox or I coupled, but before I start working with the seller of the gearbox, I would like to get the opinions of professionals first. Maybe someone had such a problem, maybe some errors in the computer have to be deleted, since another element has been installed. In the end, the computer decides whether to give a signal to the valve body. Please give me some hints. . I will be very grateful.

And was any diagnostics done before replacing the gearbox? Did the mechanic say that you need to replace it, did you run to buy another box? Since it does not drive on the "new box," maybe the electronics has failed?

The car has been standing in front of the block since September, so I had some time to consult it with a few mechanics. The box had over 320,000 driven. It's supposedly the limit of its endurance. The mechanic said that some disks were worn and when the oil heats up, they slip and do not pull. The computer also announced that there was a gearbox failure. In addition, shortly before the failure, it began to jerk when changing gears. So the decision and buying another one was not hasty. The second gearbox is also used, but as the seller assured, it was operational. I would like to get the opinion of professionals before I make a decision. Is it a mechanic's mistake when folding or maybe a sensor has not been connected. Each subsequent intervention means additional expenses, and I wonder about this strange whistle when starting the engine

Added after 5 [hours] 44 [minutes]:

Referring to the previous answer, I will add that 2 days before replacing the box, I started the car and drove it a bit forward and back, the box worked, but probably, as I mentioned, it warmed up and did not move anymore. so the electronics are rather functional

I think the new one used is dead

As I mentioned earlier, the seller assured me that the gearbox he sold to me was operational. The seller is my longtime friend who has been importing spare parts for cars from Germany for a long time. At my request, he was looking for a working box for my car for quite a long time. In the end, he found a Citroen which had the rear massacred after the accident, while the drive and chassis were functional and before he bought the boxes, he first drove this car around the square. No jerks or any problems, the gearbox was smooth, so it was operational

Mechanics often make assembly mistakes. By mounting the box to the already bolted converter. Second thing, the torque converter was also changed along with the "new" gearbox?

And the oil is there? The 4HP20 is quite simple and mean at the same time

The topic was not solved because I sold the car for scrap with a faulty gearbox, which is in the opinion of the mechanic replacing the gearbox. the car Only after getting the information in the Citroen service I found out that the gearboxes need to be trained to work with the computer in the car, just like any sensor, if it fails and we replace it, the computer still signals malfunctions. only the deletion of the error in the computer with the service tester restores normal operation of the component. The mechanic turned out to be an ordinary trickster. I already bought another car at that time and for a Citroen in a few days I would have to pay another insurance premium, so I decided to sell.