In a PLC-controlled system, problems rarely occur inside the PLC. A malfunction is nearly always on the input side or output side of the PLC. The ladder diagram shows you what the inputs and outputs are, and it shows how each output responds to a given input change. This is, in fact, what makes a ladder diagram especially useful for troubleshooting — you can follow the current flows of control circuits to find what's wrong.

The output of one control circuit is often the input to another elsewhere on the drawing. For example, at line 27 on your drawing, a limit switch is an input to a relay control circuit. The circuit ends with a coil contact (output). When tripped, the switch provides a signal and the coil picks up, thereby energizing the contact.

Now at line 104, we see the contact again. This time, it's an input to a motor starter circuit. On line 138, the starter output contact is the input to a motorized control valve that dumps raw material into a mixer.

That valve is what we call "the final control element" for this particular process function. To determine why the mixer isn't getting raw material, you'd start with the valve and trace back the circuits that determine when the motor will operate to open the valve.