What is a possible reason for a machine using a liquid level control to fail to fill?

A machine using a liquid level control can fail to fill for several reasons, and each of the options presented is a valid potential cause.

A shorted probe can disrupt the function of the liquid level control system. The probe is critical for detecting the water level; if it is shorted, it may not send accurate readings to the control system. This can prevent the machine from realizing that it needs to fill with more water, resulting in a fill failure.

A bad fill valve is another possible reason for the failure to fill. The fill valve is responsible for allowing water to enter the system. If the valve is malfunctioning, either due to blockage, mechanical failure, or electrical issues, it may not open when prompted by the control system, leading to a failure in filling the machine.

Additionally, if the water supply is turned off, the machine will obviously have no water to fill, causing it to fail to operate as intended. This situation can easily be overlooked, especially if other components seem to be functioning normally.

Each of these conditions can independently prevent the machine from filling, making "all of the above" a comprehensive choice that encompasses all likely issues contributing to the failure to fill.