Samsung SR-L629EV

Samsung SR-L629EV serial no. 602041ATA00028 B
Hello group,
My Samsung fridge freezer is 4 years old and has been giving some problems recently. Freezer was ok, but fridge would only register 9 degrees. Service Engineer from insurance co came out and said thermostat problem, easy repair. We waited 3 weeks for him to come out and fit. When I came home there was bits of fridge all over the kitchen- but no engineer. He’d said the Samsung was a write-off and the freezer was also kaput. I wasn’t best pleased about the service and complained. The ins co offered to buy us a new one (£950) as long as we paid up the insurance (£160). Which is ok.
But our written off freezer continued to work ok. I managed to take off the polystyrene insulation at the back inside the fridge (unit isolated) and found the heat exchanger inside a block of ice. I used a hair dryer and defrosted it. Then I turned the fridge back on and Bingo! Nothing worked! Even the freezer had stopped. But there was a “d” flashing on the display. I tried leaving it to stand for a while, but it didn’t come on. Fiddled about removing various connectors but still ended up with the same old “d”. So left it for 3 days, turned it on to try something else, and it came on ok by itself, with fridge and freezer working. Now its running ok, although I’m sure that the original fault must still exist. I’m just going to keep an eye on that heat exchanger in the back of the fridge.
What I want to know is this: What does “d” represent as a fault code; if one of my thermisters has failed, which is it likely to be and how much do they cost; why did it take so long for my Samsung to decide to work?