After prolonged use the element will fail causing one of the conditions described.Oven liner take longer to preheat than a cheaper oven, reason is the electronics and technology that is built into them, also the amount of metal holds more heat and for longer. In turn this makes for a more through cooking experience.

 

Testing the convection element with a volt meter will verify the element's operation. The convection element needs 220v to heat, by removing the access cover over the element you can gain access to the element to test.

The operation mode of the oven liner

Removing the back panel there are some screws on the back access panel that hold it to the oven liner. With the access panel removed you will see the convection element on the back wall. There will be several screw that mount to the oven liner. Remove the screws and pull the element out, take the element out slowly trying not to pull the wires off the element when removing. If one of the element wires come off you may need to pull the oven out to retrieve.

 

If the element has continuity then the internal function is operating fine. Testing to ensure proper voltage to the element may be need to make the proper diagnoses.These unit use thermal protection in case a overheat condition did occur. Check the serial break, on a few of these there is an update kit for the thermal limit circuit, some where resetting preventing the oven liner from heat at too low of temperature.