Edit.

Looking at the diagram again, thre are FOUR heaters.

Interestingly, TWO of them use the term "line heating". #2 is "line Heating" and #10 is "Line heating of the intake pipe in the active tank"

Did the mechanic ohm the heater element on the part he replace? Does he have the knowledge to figure out which connections to test the 'line heating' element inside the tank??