Which of the following can indicate a fault in a reluctance type speed sensor?

A weak signal output from a reluctance type speed sensor can indicate a fault because these sensors operate based on the principle of magnetic reluctance, which changes with the presence and motion of a ferromagnetic target, such as a toothed wheel. When the signal output is weaker than expected, it may suggest that the sensor is not detecting the target effectively, possibly due to issues like misalignment, debris on the sensor, or internal damage leading to a decrease in the magnetic field strength.

In contrast, high voltage output may indicate a different type of malfunction not necessarily related to the functionality of the sensor itself, while a consistent signal across all speeds could suggest that the sensor is not functioning properly, but it does not point specifically to a fault condition. Variable resistance readings could imply issues with wiring or connections rather than with the sensor itself. Thus, a weak signal output is a clear indicator of potential faults in the system, making it the correct choice.