A magnetic field sensor comprises a first coil configured to generate a magnetic field having a first frequency and induce a reflected magnetic field from a target. A second coil configured to generate a diagnostic magnetic field having a second frequency is included. The diagnostic magnetic field is configured not to induce a reflected magnetic field from the target that is measurable by the magnetic field sensor. At least two magnetic field sensing elements detect the reflected magnetic field and the diagnostic magnetic field and generate a signal representing the reflected magnetic field and the diagnostic magnetic field. A processing circuit is coupled to the at least two magnetic field sensing elements and configured to receive the signal, extract a diagnostic magnetic field portion of the signal representing the diagnostic magnetic field, and generate an error signal if a fault is detected.