Air Fryer Oven Temp. Control

Hi I have an Aobosi HL20L-GS Air Fryer oven, that has a problem displaying the correct temperature. It worked fine until the warranty expired. When I turn it on now, it shows first the (ambient) oven temperature, which is off (higher) by 19 Deg. F. Each time I use the oven, I have to adjust the off-set temperature accordingly. I would like to correct this issue. I did not open the oven yet. Can somebody suggest what to look for?