AEG Freezer defrost cycle

I have a 1994 AEG Oko Arctis 2794. Not a peep out of it until now. The problem is that the spanner symbol lights and then the temperature rises, until the temp alarm goes off, when it is warmer, we unplug the freezer and the cycle resets and the temp returns to -18. I opened the grill at the inside top and there is no icing. I suspected an electrical problem, so removed the lid. There are 2 pcb boards, One seems to be controlling mains voltage functions and the other seems to be the logic and low voltage functions. On the mains board there are 2 relays, one is a single pole, which I assume is the compressor and the other is a double pole, which I assume is the fan and heater for the defrost. This looked dodgy, due to arcing and I changed it. When I power it up, you can hear the fan and everything cools down, so it can’t be the compressor or temperature sensors. The problem seems to be that once it goes into a defrost cycle, it doesn’t reset wafterwards. Am I missing anything obvious, or am I looking at the control board. I am an electronic engineer, but have no experience with coolant, but am certain the problem isn’t in that area. I am in France and feel that the tradesmen here tend to be familiar with the refrigeration side, but not the electronics. It is a true AEG, as it hasn’t got a product number but has an E. No and an F. No and was made in Germany.