Ikea oven thermostat problems

Hello,

I’m having trouble with an Ikea 301.588.76 oven (made by Whirlpool).

The customer complained that it wasn’t heating to the correct temperature, so most likely problem is the thermostat. The Ikea parts site claims that there isn’t a thermostat in it, but as it’s sitting in front of me I don’t believe them. I have a post in the Parts board about the thermostat, but I replaced it with a similar one and it works now, but it is 50C out. Set it to 250 and it reaches 200, try any higher and there is an error. Is the problem likely to be with (I don’t know that the part’s called – a potentiometer maybe ) there’s a black plastic box that fits on the shaft with wires going to the board.

Any help would be greatly appreciated.

Grant