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.