Hey, I've got a faulty oven here that won't heat up properly. The previous owner suspected a defective heating coil, but they are all functional. I tested it with an oven thermometer: if only bottom heat is on, the thermostat switches correctly and the oven heats up as it should. However, as soon as top heat is (also) on, it only heats up to about 80°C (144°F) below the target temperature.

The sensor is located directly behind the upper coil, so it doesn't seem too far-fetched that it switches sooner in this case, but 80 degrees too early? It has been working for the past 12 years... In YouTube videos, a faulty thermostat seems to cause the stove to heat non-stop.

Any idea on how I can fix this? I don't want to buy a spare part only for the problem to persist in the end.

Edit: This is about an IKEA FRAMTID OV9 featuring an EGO 55.17253.120.