Assuming you now know which circuit that is causing the problem: have a look under the toppart. In my case the problem originated from the control PCB

After probing the PCB with a multimeter, I concluded that the relay marked 'B' must be defect. In fact, that realy was visually damaged. It looked like a spark had burned a hole in its plastic housing

Order a new relay and solder it in

Re-run the test program, and verify that the oven is no longer triggering the HFI relay

April 2015 I replaced relay B. May 2018 I replaced relay C. May 2020 I replaced relay D.