An electric heater (4) of a synthetic-material molding system (2) defines a current path (6) that includes a first measuring pickup (20), a second measuring pickup (22), a switch (16) and a third measuring pickup (24), all connected in series, wherein a heating element (14) is connected between the first measuring pickup (20) and the second measuring pickup (22). The electric heater (4) also has a diagnostic path (28) that includes a first diagnostic pickup (30), a first voltage measuring device (52), a second diagnostic pickup (32), a second voltage measuring device (54) and a third diagnostic pickup (34), all connected in series. The first measuring pickup (20) is electrically connected to the first diagnostic pickup (30) by a first protective impedance (38), the second measuring pickup (22) is electrically connected to the second diagnostic pickup (32) by a second protective impedance (40), and the third measuring pickup (24) is electrically connected to the third diagnostic pickup (34) by a third protective impedance (42).