Based on information from various sensors, the PCM controls the amount of fuel sent to the cylinder via the fuel injection system. The fuel injector is a solenoid-operated valve that meters the fuel. The PCM controls by length of time the fuel injector is held open. The PCM grounds the injector control circuit to open the injector. When the PCM energizes the injector by grounding the control circuit, the circuit voltage should be pulled low (theoretically 0V) and the fuel is injected. When the PCM de-energizes the injector by opening the control circuit, the fuel injector is closed and circuit voltage should momentarily peak before settling back to system voltage.